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

This reverts commit 5fa0b1a7.

Reason for revert:
This is not correct since external memory reporting may 1) start an incremental V8 gc which 2) invokes wrapper tracing 3) which may fail if GC is forbidden, c.f. crbug/846093 I will look into this offline.

Bug:chromium:846093, chromium:846061

Original change's description:
> [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/1069271
> Reviewed-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}

TBR=haraken@chromium.org,hpayer@chromium.org,mlippautz@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: I06899e2e5bf0c4303afdd946fd3e6a99f2954530
Reviewed-on: https://chromium-review.googlesource.com/1071288Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561467}

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

@@ -580,6 +580,8 @@ void ThreadState::ScheduleGCIfNeeded() {
   if (IsGCForbidden() || SweepForbidden())
     return;
 
+  ReportMemoryToV8();
+
   if (ShouldForceMemoryPressureGC()) {
     CompleteSweep();
     if (ShouldForceMemoryPressureGC()) {
@@ -1012,8 +1014,6 @@ void ThreadState::PostSweep() {
   DCHECK(CheckThread());
   ThreadHeap::ReportMemoryUsageForTracing();
 
-  ReportMemoryToV8();
-
   if (IsMainThread()) {
     ThreadHeapStats& stats = heap_->HeapStats();
     double collection_rate = 1.0 - stats.LiveObjectRateSinceLastGC();