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This is part 2 of 6 for FieldTrial refactoring for WebView

Part 1: https://chromium-review.googlesource.com/c/562098/
Part 2: https://chromium-review.googlesource.com/c/561920/
Part 3: https://chromium-review.googlesource.com/c/561922/
Part 4: https://chromium-review.googlesource.com/c/561980/
Part 5: https://chromium-review.googlesource.com/c/562417/
Part 6: https://chromium-review.googlesource.com/c/562021/

This CL makes the reading on the client ID happen synchronously
at WebView startup only if the enable-webview-finch command
line flag is provided. This behavior is necessary in order to be
able to set up field trials as early as possible in the WebView
startup process. GetOrCreateGUID() is now a static member of
the AwMetricsServiceClient class.

BUG=678288

Change-Id: I69153f87243fa84f6816785090bfa963e34cf0b2
Reviewed-on: https://chromium-review.googlesource.com/561920
Commit-Queue: Kyle Milka <kmilka@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Selim Gurun <sgurun@chromium.org>
Reviewed-by: Paul Miller <paulmiller@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485390}