Implement more of Flush properly. This adds support for waiting on the message
loop so we can now enforce that there is only one pending Flush at a time.
Also removes the ability to have multiple flushes pending at the same time.

TEST=covered by ppapi unit test
BUG=none
Review URL: http://codereview.chromium.org/2862002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@49899 0039d316-1c4b-4281-b951-d872f2087c98
11 files changed