Add PluginLoaderPosix tsan suppression.

tools/valgrind/tsan/suppressions.txt

 # There are a few 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 # running overnight and removing outdated bugs/suppressions.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 845 matching lines @@
   bug_116421
   ThreadSanitizer:Race
   fun:talk_base::Event::EnsureInitialized
 }
 {
   bug_116559
   ThreadSanitizer:Race
   fun:logging::::LoggingTest_Dcheck_Test::TestBody
   fun:testing::internal::HandleSehExceptionsInMethodIfSupported
 }
+{
+  bug_118345
+  ThreadSanitizer:Race
+  fun:base::Thread::message_loop
+  fun:content::BrowserThreadImpl::PostTaskHelper
+  fun:content::BrowserThread::PostTask
+  fun:PluginLoaderPosix::GetPluginsToLoad
+  fun:base::internal::RunnableAdapter::Run
+}