[heap] Report proper OOM failure if semi-space copy fails.

BUG=chromium:544774
LOG=n

Review URL: https://codereview.chromium.org/1409023004

Cr-Commit-Position: refs/heads/master@{#31400}

src/heap/mark-compact.cc

@@ -1719,8 +1719,8 @@ int MarkCompactCollector::DiscoverAndEvacuateBlackObjectsOnPage(
         if (!new_space->AddFreshPage()) {
           // Shouldn't happen. We are sweeping linearly, and to-space
           // has the same number of pages as from-space, so there is
-          // always room.
-          UNREACHABLE();
+          // always room unless we are in an OOM situation.
+          FatalProcessOutOfMemory("MarkCompactCollector: semi-space copy\n");
         }
         allocation = new_space->AllocateRaw(size, alignment);
         DCHECK(!allocation.IsRetry());

src/heap/scavenger.cc

@@ -236,7 +236,7 @@ class ScavengingVisitor : public StaticVisitorBase {
     // If promotion failed, we try to copy the object to the other semi-space
     if (SemiSpaceCopyObject<alignment>(map, slot, object, object_size)) return;
 
-    UNREACHABLE();
+    FatalProcessOutOfMemory("Scavenger: semi-space copy\n");
   }