Update from https://crrev.com/314320

tools/valgrind/valgrind_test.py

Index: tools/valgrind/valgrind_test.py
diff --git a/tools/valgrind/valgrind_test.py b/tools/valgrind/valgrind_test.py
index 5576ed3177655d43038fc6e24609227e95610000..9f132c8414a7272f423a7cb62b742ac13a7dd60d 100644
--- a/tools/valgrind/valgrind_test.py
+++ b/tools/valgrind/valgrind_test.py
@@ -380,12 +380,6 @@ class ValgrindTool(BaseTool):
 
     # The Valgrind command is constructed.
 
-    # Valgrind doesn't play nice with the Chrome sandbox.  Empty this env var
-    # set by runtest.py to disable the sandbox.
-    if os.environ.get("CHROME_DEVEL_SANDBOX", None):
-      logging.info("Removing CHROME_DEVEL_SANDBOX from environment")
-      os.environ["CHROME_DEVEL_SANDBOX"] = ''
-
     # Handle --indirect_webkit_layout separately.
     if self._options.indirect_webkit_layout:
       # Need to create the wrapper before modifying |proc|.
@@ -718,6 +712,9 @@ class DrMemory(BaseTool):
     # disable leak scan for now
     proc += ["-no_count_leaks", "-no_leak_scan"]
 
+    # disable warnings about unaddressable prefetches
+    proc += ["-no_check_prefetch"]
+
     # crbug.com/413215, no heap mismatch check for Windows release build binary
     if common.IsWindows() and "Release" in self._options.build_dir:
         proc += ["-no_check_delete_mismatch"]