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We checkout the Android WebView glue layer (frameworks/webview)
into a new third_party directory (android_webview_glue)
which is checked out via gclient.

This is built against a jar containing frameworks/base
(in android_platform/webview/frameworks.jar).

We have to work around a few other issues:
    - Change build/java_apk.gypi to bypass the hardcoded
      'src' path.
    - Move libwebviewchromium to an include so we can make both
      the prefixed and un-prefixed version in the same build.

BUG=391488
R=mkosiba@chromium.org

Review URL: https://codereview.chromium.org/477903003

Cr-Commit-Position: refs/heads/master@{#293921}