PPC: [heap] Make write barrier safe for the concurrent marker.

Port bd3d091d

Original Commit Message:

    With concurrent marking the write barrier should trigger even if the
    object is black because the concurrent marker could have fetched
    object field before marking the object black.

R=ulan@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=chromium:694255
LOG=N

Change-Id: I3e3b5b467ab3c2eca45ac8d85523c8af4f5f5d4b
Reviewed-on: https://chromium-review.googlesource.com/535736Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Jaideep Bajwa <bjaideep@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#45956}

src/ppc/code-stubs-ppc.cc

@@ -2302,9 +2302,10 @@ void RecordWriteStub::InformIncrementalMarker(MacroAssembler* masm) {
 void RecordWriteStub::CheckNeedsToInformIncrementalMarker(
     MacroAssembler* masm, OnNoNeedToInformIncrementalMarker on_no_need,
     Mode mode) {
-  Label on_black;
   Label need_incremental;
   Label need_incremental_pop_scratch;
+#ifndef V8_CONCURRENT_MARKING
+  Label on_black;
 
   // Let's look at the color of the object:  If it is not black we don't have
   // to inform the incremental marker.
@@ -2319,6 +2320,7 @@ void RecordWriteStub::CheckNeedsToInformIncrementalMarker(
   }
 
   __ bind(&on_black);
+#endif
 
   // Get the value from the slot.
   __ LoadP(regs_.scratch0(), MemOperand(regs_.address(), 0));