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Use RunLoop::Run() instead of RunLoop::RunUntilIdle() in FileSystemOperationImplTest.

RunLoop::Run() is needed if any of the operation is implemented using
asynchronous I/O in OS layer, or performed in a separate thread.
Currently, only Copy needs RunLoop::Run().

This CL replace all RunUntilIdle() with Run() to keep all test consistent.

BUG=411153
TEST=content_unittest

Review URL: https://codereview.chromium.org/557273002

Cr-Commit-Position: refs/heads/master@{#294365}