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Implement VariationsSeedService in the WebView services process to serve
variations seeds to apps. Implement VariationsSeedLoader to load seeds stored
in the app, and periodically request new seeds from the service, on
WebView startup.

VariationsSeedService replaces AwVariationsConfigurationService. The
remaining AwVariations* functionality (downloading seeds from the remote
variations server into the local WebView service, and serializing the seeds
to and from flash) will be replaced in a separate commit.

Downloading and serializing variations seeds is not yet implemented for
WebView, so use dummy seeds for now (3 KB of zeros, simulating the size
of an actual WebView variations seed).

Enable the new code via either the existing "enable-webview-variations"
flag or the "finch-exp" file. This is an empty file created by AGSA to
indicate we are in the variations experiment.

Run the new tests like so:
$ out/*/bin/run_webview_instrumentation_test_apk -f org.chromium.android_webview.\*.Variations\*

Design document:
https://docs.google.com/document/d/1vLLDfaJb6DQz1RQFthse4i8N_r1wLiVOcLE05i_9KAI/edit?usp=sharing

BUG=733857

Change-Id: I1171f7b82276c32ac53ebc1b2d4bec9ed1cfb31d
Reviewed-on: https://chromium-review.googlesource.com/954058Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Paul Miller <paulmiller@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546205}