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This implements support library callbacks (WebViewclientCompat). This
creates a new layer (support_library/callback) which the glue layer
depends on. This dependency lets us instantiate a
SupportLibWebViewContentsClientAdapter inside setWebViewClient(), and
benefit from the glue layer's parameter cleanup code (e.g., in
onReceivedError2()).

The support_library/callback glue must be a separate layer from
support_library/ glue, as that already depends on the webkit glue to
initiate state. Implementing callbacks as a separate target avoids the
circular dependency.

This refactors the (post-L) glue layer callback methods to take the
following precedence:

 1. SupportLibWebViewContentsClientAdapter (if it supports the callback)
 2. WebViewClient (if on the appropriate platform level)
 3. Default behavior (implementation provided by the glue layer)

This implements both category 1 and 2 APIs.

Design doc: http://go/wv-support-library-callbacks

Bug: 781764
Test: manual - built test application with latest support-lib changes
Change-Id: I21e28493873e670cfd428c7eeef12b0e212aeec4
Reviewed-on: https://chromium-review.googlesource.com/989015Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Gustav Sennton <gsennton@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548538}