Add base/md5* to ThreadSanitizer ignore list.

MD5 computation is a series of many tiny function calls, which causes tsan to slow down.

For example, MD5 computation accounted for ~38% of samples when running media_unittests PipelineIntegrationTest.BasicPlaybackHashed.

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/10806096

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148596 0039d316-1c4b-4281-b951-d872f2087c98

tools/valgrind/tsan/ignores.txt

@@ -65,6 +65,11 @@ fun:*base*subtle*NoBarrier_Load*
 # Keep some mangling so we don't match NoBarrier_AtomicIncrement
 fun:*base*subtle23Barrier_AtomicIncrement*
 
+# MD5 computations are very slow due since sums are computed by
+# repeatedly calling tiny functions and is unlikely to race with
+# anything.
+src:*base/md5*
+
 # Don't instrument tcmalloc
 src:*/tcmalloc/*