[oilpan] Move CompleteSweep() to ScheduleGCIfNeeded() during allocation to play safe.

CompleteSweep() needs to be called before we call ReportMemoryToV8(). This change
makes this dependency more explicit.

Change-Id: I044af54bcfdc8a7457edbb492fba6bfe4ff210aa
Reviewed-on: https://chromium-review.googlesource.com/1069271Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560987}

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

@@ -564,8 +564,6 @@ void ThreadState::ScheduleGCIfNeeded() {
   if (IsGCForbidden() || SweepForbidden())
     return;
 
-  ReportMemoryToV8();
-
   if (ShouldForceMemoryPressureGC()) {
     CompleteSweep();
     if (ShouldForceMemoryPressureGC()) {
@@ -998,6 +996,8 @@ void ThreadState::PostSweep() {
   DCHECK(CheckThread());
   ThreadHeap::ReportMemoryUsageForTracing();
 
+  ReportMemoryToV8();
+
   if (IsMainThread()) {
     ThreadHeapStats& stats = heap_->HeapStats();
     double collection_rate = 1.0 - stats.LiveObjectRateSinceLastGC();