Do not build libpixman with MSan instrumentation.

Partial revert of r244764.

BUG=334351
TBR=glider@chromium.org

Review URL: https://codereview.chromium.org/138673003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244785 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244785