Update README for AOSP WebView prebuilts.

Update the README file as it's become outdated over time:

- Remove references to specific versions of Chromium/Android, and
  instead make the instructions generic so that this file doesn't become
  stale so frequently.

- Explain that it's generally not desirable to rebuild the same version,
  and that the latest stable version should be used in most cases.

- Explain in more detail the issue with building against the latest SDK.

Bug: 111654347
Test: n/a
Change-Id: I0521939dc323b6a8723d1f3d90e7441c8068e23a

1 files changed, 36 insertions, 25 deletions

diff --git a/README b/README
index 19e5378..e4d341b 100644
--- a/README
+++ b/README
@@ -4,31 +4,42 @@ now be built entirely from the Chromium source code.
 General instructions for building WebView from Chromium:
 https://www.chromium.org/developers/how-tos/build-instructions-android-webview
 
-------
-
-The prebuilt libwebviewchromium.so included in these APKs is built from Chromium
-release tag 61.0.3163.98, using the GN build tool. To match our build settings, set:
-
-target_os="android"
-is_debug=false
-is_official_build=true
-is_chrome_branded=false
-use_official_google_api_keys=false
-exclude_unwind_tables=true
-enable_resource_whitelist_generation=true
-ffmpeg_branding="Chrome"
-proprietary_codecs=true
-enable_remoting=true
-
-in your GN argument file before building.
-
-------
-
-Due to WebView API changes in the O-MR1 release, the Java code in the Chromium
-3163 branch is not compatible with O-MR1. We'll be working on upstreaming
-the O-MR1 specific Java changes to Chromium once the final O-MR1 SDK is released.
+For questions about building WebView, please see
+https://groups.google.com/a/chromium.org/forum/#!forum/android-webview-dev
 
 ------
 
-For questions about building WebView, please see
-https://groups.google.com/a/chromium.org/forum/#!forum/android-webview-dev
+The prebuilt APKs here are built from Chromium upstream sources; check the
+commit messages to see the version number for a particular prebuilt (the version
+number looks like 74.0.3729.127 and this is also the name of the tag in the
+Chromium git repository).
+
+If you want to build your own WebView, you should build the latest stable
+version, not the version published here: newer versions have important security
+and stability improvements.
+
+However, if you want to reproduce the native library (libwebviewchromium.so)
+contained in these prebuilt APKs, you should be able to do so with the following
+GN arguments:
+
+target_os = "android"
+is_debug = false
+is_component_build = false
+is_official_build = true
+is_chrome_branded = false
+use_official_google_api_keys = false
+exclude_unwind_tables = true
+ffmpeg_branding = "Chrome"
+proprietary_codecs = true
+enable_remoting = true
+
+as well as specifying the appropriate target_cpu, which should be one of
+"arm", "arm64", "x86" or "x64".
+
+To build a complete WebView APK it is necessary to also set android_sdk_release
+to the current Android SDK version (for example, "q"). However, it's not always
+possible to do this successfully for all public Chromium versions: we are not
+able to publish the required Java code changes to support a given Android
+version until after that Android version's SDK has been publicly released, and
+so the public versions of Chromium do not always support the latest version of
+Android.