[base] Make the current platform accessor atomic.

TracingCpuProfiler test updates the current plaform while
concurrent marking is running.

This patch also disables stress-incremental-marking for
mjsunit/regress-430201.

BUG=chromium:694255

Change-Id: I85ff538c47bce0300cde3204989ef3f9512b805f
Reviewed-on: https://chromium-review.googlesource.com/533873Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45921}

src/v8.cc

@@ -6,6 +6,7 @@
 
 #include "src/api.h"
 #include "src/assembler.h"
+#include "src/base/atomicops.h"
 #include "src/base/once.h"
 #include "src/base/platform/platform.h"
 #include "src/bootstrapper.h"
@@ -109,13 +110,16 @@ void V8::ShutdownPlatform() {
 
 
 v8::Platform* V8::GetCurrentPlatform() {
-  DCHECK(platform_);
-  return platform_;
+  v8::Platform* platform = reinterpret_cast<v8::Platform*>(
+      base::Relaxed_Load(reinterpret_cast<base::AtomicWord*>(&platform_)));
+  DCHECK(platform);
+  return platform;
 }
 
-
-void V8::SetPlatformForTesting(v8::Platform* platform) { platform_ = platform; }
-
+void V8::SetPlatformForTesting(v8::Platform* platform) {
+  base::Relaxed_Store(reinterpret_cast<base::AtomicWord*>(&platform_),
+                      reinterpret_cast<base::AtomicWord>(platform));
+}
 
 void V8::SetNativesBlob(StartupData* natives_blob) {
 #ifdef V8_USE_EXTERNAL_STARTUP_DATA

test/mjsunit/regress/regress-430201.js

@@ -25,7 +25,7 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// Flags: --allow-natives-syntax --expose-gc
+// Flags: --allow-natives-syntax --expose-gc --nostress-incremental-marking
 
 var array_1 = [];