add suppression for uninit error from content::*::NotifyPluginProcessHostHelper

TBR=jam@chromium.org
BUG=372254

Review URL: https://codereview.chromium.org/279133002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269736 0039d316-1c4b-4281-b951-d872f2087c98

tools/valgrind/drmemory/suppressions_full.txt

@@ -2160,3 +2160,7 @@ name=bug_372226
 *!WebCore::PinchViewport::reset
 *!WebCore::PinchViewport::PinchViewport
 *!WebCore::FrameHost::FrameHost
+
+UNINITIALIZED READ
+name=bug_372254
+*!content::*::NotifyPluginProcessHostHelper