Move core/platform/audio into blink_platform.dll

This CL moves all the files in core/platform/audio into blink_platform.dll.
There's stil a bit more massaging left to remove a few instances of the word
"chromium," but I'll handle those in a followup CL.

This CL doesn't change any behavior. It just moves files.

R=kbr@chromium.org, rtoy@chromium.org
BUG=297477

Review URL: https://codereview.chromium.org/26327004

git-svn-id: svn://svn.chromium.org/blink/trunk@159093 bbb929c8-8fbe-4397-9dbb-9b2b20218538