Fix expectation in PipelineIntegrationTestBase::Seek().

Fix expectation in PipelineIntegrationTestBase::Seek().

In Pipeline, we satisfy seek callback as soon as Flush() is finished.
After that, we call StartPlayingFrom(time) which will trigger a buffering
state change.

In PipelineIntegrationTestBase::Seek(), we quit the messageloop right
after Seek() callback is fired. So if we don't run the message loop again
in the test (like we did in FFmpegRegressionTest, where we do a seek at
the end of a test), OnBufferingStateChanged expectation won't be
satisfied.

BUG=430691,426560
TEST=Both PieplineIntegrationTest and FFmpegRegressionTest pass.

Review URL: https://codereview.chromium.org/692753005

Cr-Commit-Position: refs/heads/master@{#302956}