Defined the base interface SeekableBuffer and a thread safe implemntation of
ThreadSafeSeekableBuffer. This class is to be used for media resource loading
that does the meat of buffer queueing and handles short seeking for short distance
out-of-order read patterns.

Review URL: http://codereview.chromium.org/113213

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@16348 0039d316-1c4b-4281-b951-d872f2087c98