[heap] Minor MC: Allow modifying code

Similar to the Scavenger, the Minor MC updates typed pointers into the
young generation and thus requires priviliges to write to code space.

Bug: chromium:651354
Change-Id: If5ce600f651ccb04b6ac3d82e6c2e311bfd7dc4d
Reviewed-on: https://chromium-review.googlesource.com/809013Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49884}

src/heap/heap.cc

@@ -1762,6 +1762,7 @@ void Heap::MinorMarkCompact() {
   PauseAllocationObserversScope pause_observers(this);
   IncrementalMarking::PauseBlackAllocationScope pause_black_allocation(
       incremental_marking());
+  CodeSpaceMemoryModificationScope code_modifcation(this);
 
   minor_mark_compact_collector()->CollectGarbage();