Remove unused includes & using statements from blink::Platform implementations.

Review URL: https://codereview.chromium.org/161873002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250945 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250945
Cast: end2end bug fixes

main changes:
- Updating synthetic video test sequence to be more uniform.
- Fixing added delay to render/playout times introduced in the rtp-stats API update.
- Sending audio packets to pacer in a timed manner.
 

BUG=329563

Review URL: https://codereview.chromium.org/147993010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250947 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250947