Create less segments in std::string internals

On my unit_tests runs, up to 25% of the memory accesses were from inside
the std::string internals, which is A LOT.
TBR=glider
TEST=unit_tests should become faster
Review URL: https://chromiumcodereview.appspot.com/10834050

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148748 0039d316-1c4b-4281-b951-d872f2087c98

tools/valgrind/tsan/ignores.txt

@@ -47,6 +47,11 @@ fun:*_M_is_shared*
 fun:*_M_set_leaked*
 fun:*_M_set_sharable*
 
+# Comparison of std::strings sometimes takes a lot of time but we don't really
+# need precise stack traces there.
+fun_hist:_ZStltIcSt11char_traitsIcESaIcEEbRKSbIT_T0_T1_ES8_
+fun_hist:_ZNKSs7compareERKSs
+
 # Don't instrument intercepts
 src:*ts_valgrind_intercepts.c