heap: Aggressively reduce memory on testing GCs

This restores previous state where we would (a) discard pages, and (b)
compact on testing GCs that run without stack.

Bug: chromium:1079150, chromium:1082136, chromium:1077663
Change-Id: I53fa1e97a2537aa93d41772ace1f1c803deaa342
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2199152Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768711}

third_party/blink/renderer/platform/heap/heap_compact.cc

@@ -357,7 +357,8 @@ bool HeapCompact::ShouldCompact(BlinkGC::StackState stack_state,
 
   // Only enable compaction when in a memory reduction garbage collection as it
   // may significantly increase the final garbage collection pause.
-  if (reason == BlinkGC::GCReason::kUnifiedHeapForMemoryReductionGC) {
+  if (reason == BlinkGC::GCReason::kUnifiedHeapForMemoryReductionGC ||
+      reason == BlinkGC::GCReason::kUnifiedHeapForcedForTestingGC) {
     return free_list_size_ > kFreeListSizeThreshold;
   }
 

third_party/blink/renderer/platform/heap/thread_state.h

@@ -276,7 +276,9 @@ class PLATFORM_EXPORT ThreadState final {
   // Returns true if the current GC is a memory reducing GC.
   bool IsMemoryReducingGC() const {
     return current_gc_data_.reason ==
-           BlinkGC::GCReason::kUnifiedHeapForMemoryReductionGC;
+               BlinkGC::GCReason::kUnifiedHeapForMemoryReductionGC ||
+           current_gc_data_.reason ==
+               BlinkGC::GCReason::kUnifiedHeapForcedForTestingGC;
   }
 
   bool IsUnifiedHeapGC() const {