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Now, TestingPlatformSupport registers itself as the current Platform
instance, but it is inside the constructor.

On the other hand, a child class TestingPlatformSupportWithMockScheduler
expects the parent class reigsters it for children.

But, to register a Platform instance, virtual methods should be ready
to call. Rephrasing it, we can not register a Platform instance
correctly inside a super-class's constructor.

This patch provides ScopedTestingPlatformSupport class template that
manages TestingPlatformSupport instance and Platform instance
installation.

Without this patch, Platform::current()->mainThread() returns a
wrong WebThread, and it makes new tests that I will add fail.

BUG=n/a
TEST=platform_blink_unittests, webkit_unit_tests

Review-Url: https://codereview.chromium.org/2588403002
Cr-Commit-Position: refs/heads/master@{#443219}