WebViewImpl: Clear WebViewScheduler on Close().

We plan to have WebViewScheduler owned by Page, which will delay the
execution of WebViewScheduler's destructor because Page is garbage
collected.

This patch moves the execution of the destructor to WebViewImpl::
Close() to ensure nothing will break even if we clear the
WebViewScheduler object early.

Bug: 812986
Change-Id: I5de6b7ad3f3598922a408b2a26cec4718a132f6f
Reviewed-on: https://chromium-review.googlesource.com/923246Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537269}

third_party/WebKit/Source/core/exported/WebViewImpl.cpp

@@ -1607,6 +1607,8 @@ void WebViewImpl::Close() {
     page_.Clear();
   }
 
+  scheduler_.reset();
+
   // Reset the delegate to prevent notifications being sent as we're being
   // deleted.
   client_ = nullptr;

third_party/WebKit/Source/core/exported/WebViewImpl.h

@@ -676,7 +676,7 @@ class CORE_EXPORT WebViewImpl final
 
   WebPageImportanceSignals page_importance_signals_;
 
-  const std::unique_ptr<WebViewScheduler> scheduler_;
+  std::unique_ptr<WebViewScheduler> scheduler_;
 
   // TODO(lfg): This is used in order to disable compositor visibility while
   // the page is still visible. This is needed until the WebView and WebWidget