Avoid threading races on TraceSamplingThread's members.

See a TSAN log in the bug for more details.

BUG=305459

Review URL: https://codereview.chromium.org/26541005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228889 0039d316-1c4b-4281-b951-d872f2087c98
Exclude emulation prevention bytes in slice_param.slice_data_bit_offset

See http://lists.freedesktop.org/archives/libva/2012-March/000793.html
for the comment about the assumption of slice_data_bit_offset.

BUG=chromium:301744
TEST=play the problematic video in chrome.

Review URL: https://codereview.chromium.org/27006002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228890 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228890