Create less segments in std::string internals

tools/valgrind/tsan/ignores.txt

 # This file lists the functions, object files and source files
 # which should be ignored (i.e. not instrumented) by ThreadSanitizer.
 # See http://code.google.com/p/data-race-test/wiki/ThreadSanitizerIgnores.
 
 # ignore these libraries
 obj:*/libfreetype*
 obj:*/libdbus*
 
 # we ignore the whole NSS library for now since
 # its instrumentation is very slow.
@@ skipping 29 matching lines @@
 
 # False reports on std::string internals, see
 # http://code.google.com/p/data-race-test/issues/detail?id=40
 fun:*_M_mutateE*
 fun_r:*_M_set_length_and_sharable*
 fun:*_M_is_leaked*
 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
 
 ##################################################################
 # Don't instrument synchronization code
 src:*base/threading/thread_local_storage*
 src:*base/stats_counters*
 src:*base/synchronization/condition_variable*
 src:*base/synchronization/lock*
 src:*base/synchronization/waitable_event*
@@ skipping 98 matching lines @@
 fun:__gen_tempname
 
 # The sqlite cache is racing against a few different stacktraces,
 # so let's ignore it recursively. See http://crbug.com/84094
 fun_r:pcache1Fetch
 
 # "Suppress" a data race in TraceLog::GetCategory which has
 # fun:MessageLoop::RunTask at the top of the "current" stack which we don't want
 # to suppress. See http://crbug.com/98926
 fun:*base*debug*TraceLog*GetCategoryInternal*