Revert "Call blink::WebMemoryCoordinator::OnPurgeMemory in RenderThreadImpl::ReleaseFreeMemory"

This reverts commit f4544427.

Reason for revert: speculative fix for https://bugs.chromium.org/p/chromium/issues/detail?id=840988&can=2&start=0&num=100&q=owner%3Ame&colspec=ID%20Pri%20M%20Stars%20ReleaseBlock%20Component%20Status%20Owner%20Summary%20OS%20Modified&groupby=&sort=&hotlist_id=

Original change's description:
> Call blink::WebMemoryCoordinator::OnPurgeMemory in RenderThreadImpl::ReleaseFreeMemory
> 
> |WebMemoryCoordinator::OnPurgeMemory| has been only called in |RenderThreadImpl::OnPurgeMemory|,
> and |RenderThreadImpl::OnPurgeMemory| is only able to be called by |base::MemoryCoordinatorClient|.
> But, if there is no performance degradation, it would be good to purge used memory as much as
> possible. |RenderThreadImpl::ReleaseFreeMemory| has been more called than |RenderThreadImpl::OnPurgeMemory|
> from some callers. So we can free up more memory if we move |WebMemoryCoordinator::OnPurgeMemory|
> from |RenderThreadImpl::OnPurgeMemory| to |RenderThreadImpl::ReleaseFreeMemory|.
> 
> TEST: Covered RenderThreadImplDiscardableMemoryBrowserTest.ReleaseFreeMemory
> 
> Bug: None
> Change-Id: I225401bcd18f236265305aaf5eaf4b4219d9cde4
> Reviewed-on: https://chromium-review.googlesource.com/1049414
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Commit-Queue: Gyuyoung Kim <gyuyoung.kim@lge.com>
> Cr-Commit-Position: refs/heads/master@{#556728}

TBR=haraken@chromium.org,gyuyoung.kim@lge.com

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

Bug: None
Change-Id: I26910af177818011bf1aa00efeacb9bb8058f108
Reviewed-on: https://chromium-review.googlesource.com/1052987Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Robert Kroeger <rjkroege@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557322}

content/renderer/render_thread_impl.cc

@@ -2377,6 +2377,8 @@ void RenderThreadImpl::OnPurgeMemory() {
 
   OnTrimMemoryImmediately();
   ReleaseFreeMemory();
+  if (blink_platform_impl_)
+    blink::WebMemoryCoordinator::OnPurgeMemory();
 }
 
 void RenderThreadImpl::RecordPurgeMemory(RendererMemoryMetrics before) {
@@ -2510,7 +2512,6 @@ void RenderThreadImpl::ReleaseFreeMemory() {
     // Purge Skia font cache, resource cache, and image filter.
     SkGraphics::PurgeAllCaches();
     blink::DecommitFreeableMemory();
-    blink::WebMemoryCoordinator::OnPurgeMemory();
   }
 }