Enable TCMalloc on Linux by default.

tools/valgrind/memcheck/suppressions.txt

 # There are three kinds of suppressions in this file.
 # 1. third party stuff we have no control over
 #
 # 2. intentional unit test errors, or stuff that is somehow a false positive
 # in our own code, or stuff that is so trivial it's not worth fixing
 #
 # 3. Suppressions for real chromium bugs that are not yet fixed.
 # These should all be in chromium's bug tracking system (but a few aren't yet).
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 328 matching lines @@
    fun:pthread_create@@GLIBC_2.1
 }
 {
    leak_in_ps
    Memcheck:Leak
    fun:malloc
    fun:nss_parse_service_list
    ...
    obj:/bin/ps
 }
+{
+   # Valgrind issues with tcmalloc's VDSOSupport module
+   bug_28149a
+   Memcheck:Addr1
+   fun:_ZN4base11VDSOSupport11ElfMemImage4InitEPKv
+   fun:_ZN4base11VDSOSupport11ElfMemImageC1EPKv
+   fun:_ZN4base11VDSOSupportC1Ev
+   fun:_ZN4base11VDSOSupport4InitEv
+   fun:_ZN4base14VDSOInitHelperC1Ev
+   fun:_Z41__static_initialization_and_destruction_0ii
+   fun:_GLOBAL__I__ZN4base11VDSOSupport12kInvalidBaseE
+   obj:/b/slave/webkit-rel-linux-valgrind-layout/build/src/sconsbuild/Release/test_shell
+   obj:/b/slave/webkit-rel-linux-valgrind-layout/build/src/sconsbuild/Release/test_shell
+   fun:__libc_csu_init
+   fun:(below main)
+}
+{
+   # Valgrind issues with tcmalloc's VDSOSupport module
+   bug_28149b
+   Memcheck:Addr4
+   ...
+   fun:_ZNK4base11VDSOSupport12LookupSymbolEPKcS2_iPNS0_10SymbolInfoE
+   ...
+   fun:_Z41__static_initialization_and_destruction_0ii
+   ...
+}
 
 #-----------------------------------------------------------------------
 # 2. intentional unit test errors, or stuff that is somehow a false positive
 # in our own code, or stuff that is so trivial it's not worth fixing
 
 {
    logging::InitLogging never frees filename. It would be hard to free properly.
    Memcheck:Leak
    ...
    fun:_ZN7logging11InitLoggingEPKcNS_18LoggingDestinationENS_15LogLockingStateENS_20OldFileDeletionStateE
@@ skipping 1281 matching lines @@
    fun:_ZN8chromeos14NetworkLibrary24NetworkTrafficTimerFiredEv
    fun:_Z16DispatchToMethodIN8chromeos14NetworkLibraryEMS1_FvvEEvPT_T0_RK6Tuple0
    fun:_ZN4base9BaseTimerIN8chromeos14NetworkLibraryELb0EE9TimerTask3RunEv
    fun:_ZN11MessageLoop7RunTaskEP4Task
    fun:_ZN11MessageLoop21DeferOrRunPendingTaskERKNS_11PendingTaskE
    fun:_ZN11MessageLoop13DoDelayedWorkEPN4base4TimeE
    fun:_ZN4base19MessagePumpLibevent3RunEPNS_11MessagePump8DelegateE
    fun:_ZN11MessageLoop11RunInternalEv
    fun:_ZN11MessageLoop10RunHandlerEv
 }