android_webview: support building a stub WebView.

Allwo building a WebView APK which doesn't contain any code or assets,
and no resources other than the icon. This APK has a special manifest
tag which will direct the system to provide the missing files from
another "donor" package at runtime that has compatible code (this will
be Monochrome).

To enable this to work, a new APK variable is defined called
"generate_buildconfig_java" so that the BuildConfig.java file can be
omitted from this APK (leaving the generated classes.dex containing
nothing but the AAPT-generated R class for the single icon resource).
The runtime doesn't like having multiple APKs in the classpath which
define the same classes, so we need to omit this BuildConfig to avoid
clashing with Monochrome's.

BUG=664456

Review-Url: https://codereview.chromium.org/2634563002
Cr-Original-Commit-Position: refs/heads/master@{#453911}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: cbafd406aff624f7191f76d64e3c0fcb66b60653

config/android/rules.gni

@@ -1464,6 +1464,9 @@ if (enable_java_templates) {
   #   testonly: Marks this target as "test-only".
   #   write_asset_list: Adds an extra file to the assets, which contains a list of
   #     all other asset files.
+  #   generate_buildconfig_java: If defined and false, skip generating the
+  #     BuildConfig java class describing the build configuration. The default
+  #     is true for non-test APKs.
   #   requires_sdk_api_level_23: If defined and true, the apk is intended for
   #     installation only on Android M or later. In these releases the system
   #     linker does relocation unpacking, so we can enable it unconditionally.
@@ -1675,6 +1678,10 @@ if (enable_java_templates) {
         defined(invoker.create_density_splits) && invoker.create_density_splits
     _create_language_splits =
         defined(invoker.language_splits) && invoker.language_splits != []
+    _generate_buildconfig_java = !defined(invoker.apk_under_test)
+    if (defined(invoker.generate_buildconfig_java)) {
+      _generate_buildconfig_java = invoker.generate_buildconfig_java
+    }
 
     # Help GN understand that _create_abi_split is not unused (bug in GN).
     assert(_create_abi_split || true)
@@ -1835,7 +1842,7 @@ if (enable_java_templates) {
       _srcjar_deps += [ ":${_template_name}__native_libraries_java" ]
     }
 
-    if (!defined(invoker.apk_under_test)) {
+    if (_generate_buildconfig_java) {
       java_cpp_template("${_template_name}__build_config_java") {
         package_name = "org/chromium/base"
         sources = [