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This moves functions defined in web_task_runner.h to two different
headers in platform/scheduler/public: post_cancellable_task.h and
post_cross_thread_task.h. The header web_task_runner.h remains in
the same place, just having the forwarding #include statements, so
the existing code wouldn't break.

Minor adjustments are made to the moved code, like: (1) using
PLATFORM_EXPORT rather than BLINK_PLATFORM_EXPORT, (2) using
base::TimeDelta instead of WTF::TimeDelta, and (3) revising comments.

web_task_runner_test.cc is also moved, and is simply renamed to
post_cancellable_task_unittest.cc because it only contains tests for
PostCancellableTask().

Bug: 826203
Change-Id: I1fa4f35d278975fbca1a7c27fecf37342b0c76c8
Reviewed-on: https://chromium-review.googlesource.com/c/1339503Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608732}