GpuVideoDecoder: avoid deleting textures in use by client

PictureBuffers (and their backing textures) held in dismissed_picture_buffers_
should not be destroyed until the GpuVideoDecoder is itself destroyed, as
clients to the GpuVideoDecoder may still be using them.

BUG=None
TEST=local build, run on CrOS snow, build on desktop Linux

Review URL: https://codereview.chromium.org/26273007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228878 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228878