Fix GcStress test failure.

CL "[Cleanup] String::GetChars() should assert against heap allocation"
was missing one DisallowHeapAllocation declaration.

(I had Michi look over the change, as the callstack actually comes from
 the garbage collector. Marja, I put you on TBR).

Thanks all..


TBR=marja@chromium.org

No-tree-checks: true
No-try: true
Bug: v8:8238
Change-Id: I71333124bc4bcef945430fc5242a516b6ed277ff
Reviewed-on: https://chromium-review.googlesource.com/c/1351013
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57839}

src/parsing/scanner-character-streams.cc

@@ -358,6 +358,7 @@ class RelocatingCharacterStream
   }
 
   void UpdateBufferPointers() {
+    DisallowHeapAllocation no_gc;
     Range<uint16_t> range = byte_stream_.GetDataAt(0, runtime_call_stats());
     if (range.start != buffer_start_) {
       buffer_cursor_ = (buffer_cursor_ - buffer_start_) + range.start;