content: Update the includes paths of WebAudioSourceProvider.h to its new location.

This header file moved to public/platform/ in Blink code base.
After this lands, the forwarding header in public/web/ will be removed from Blink.

BUG=None
TEST=None, no functional changes.
R=fischman@chromium.org

Review URL: https://codereview.chromium.org/26169002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227394 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227394