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/1049414Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung.kim@lge.com>
Cr-Commit-Position: refs/heads/master@{#556728}

content/renderer/render_thread_impl.cc

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