Makes sure command buffer's CopyTextureCHROMIUM doesn't leak uninitialized textures.

BUG=322770
TEST=context-attribute-preserve-drawing-buffer.html on mac
R=bajones@chromium.org

Review URL: https://codereview.chromium.org/133443012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249584 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249584