Reland: Call blink::WebMemoryCoordinator::OnPurgeMemory in RenderThreadImpl::ReleaseFreeMemory

Orignal commit: f4544427

This CL was reverted because it caused the timeout test failure on fyi bots.
But, it looks like the issue was solved. So now this CL lands original CL without
a duplicated function invocation.

Bug: 842019
Test: Covered by RenderThreadImplDiscardableMemoryBrowserTest.ReleaseFreeMemory
Change-Id: I52a4bafc1688822a11fa2408ad7422331fa62a46
Reviewed-on: https://chromium-review.googlesource.com/1084369Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung.kim@lge.com>
Cr-Commit-Position: refs/heads/master@{#564331}

content/renderer/render_thread_impl.cc

@@ -2333,8 +2333,6 @@ void RenderThreadImpl::OnPurgeMemory() {
 
   OnTrimMemoryImmediately();
   ReleaseFreeMemory();
-  if (blink_platform_impl_)
-    blink::WebMemoryCoordinator::OnPurgeMemory();
 }
 
 void RenderThreadImpl::RecordPurgeMemory(RendererMemoryMetrics before) {
@@ -2470,7 +2468,7 @@ void RenderThreadImpl::ReleaseFreeMemory() {
   if (blink_platform_impl_) {
     // Purge Skia font cache, resource cache, and image filter.
     SkGraphics::PurgeAllCaches();
-    blink::DecommitFreeableMemory();
+    blink::WebMemoryCoordinator::OnPurgeMemory();
   }
 }