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With this CL the gin platform implements the new v8::Platform API
functions {GetForegroundTaskRunner} and {GetBackgroundTaskRunner}.

The existing task posting function of the V8Platform now also use these
taskrunners to post tasks. Thereby we can move some of the
implementation from the V8Platform and the PerIsolateData to the new
taskrunners.

R=rmcilroy@chromium.org

Change-Id: I27ea6dd5f6ef8409e394f23ad914f468c09ded4a
Reviewed-on: https://chromium-review.googlesource.com/758578
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517067}