Eagerly force GC in a few cctests to avoid spurious nosnap failures

See this v8-dev thread for more background:
https://groups.google.com/d/msg/v8-dev/71PVa9fMzoc/AvBZ9Fw4BQAJ

TBR=hpayer@chromium.org

Change-Id: I49957a72c8faec4cf5032cbf6437a461d7139b83
Reviewed-on: https://chromium-review.googlesource.com/486062Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44824}

test/cctest/heap/test-spaces.cc

@@ -637,6 +637,12 @@ UNINITIALIZED_TEST(InlineAllocationObserverCadence) {
 
     Isolate* i_isolate = reinterpret_cast<Isolate*>(isolate);
 
+    // Clear out any pre-existing garbage to make the test consistent
+    // across snapshot/no-snapshot builds.
+    i_isolate->heap()->CollectAllGarbage(
+        i::Heap::kFinalizeIncrementalMarkingMask,
+        i::GarbageCollectionReason::kTesting);
+
     NewSpace* new_space = i_isolate->heap()->new_space();
 
     Observer observer1(512);

test/cctest/test-unboxed-doubles.cc

@@ -916,6 +916,10 @@ TEST(Regress436816) {
   Factory* factory = isolate->factory();
   v8::HandleScope scope(CcTest::isolate());
 
+  // Force a GC to free up space before we allocate objects whose
+  // mid-test states would fail heap verification.
+  CcTest::CollectAllGarbage(i::Heap::kFinalizeIncrementalMarkingMask);
+
   const int kPropsCount = kSmiValueSize * 3;
   TestPropertyKind props[kPropsCount];
   for (int i = 0; i < kPropsCount; i++) {