Fix WebViewPlugin::scheduleAnimation crash

The crash was probably due to accessing a dangling pointer to the plugin
container during a small time frame between PepperWebPluginImpl::destroy()
and the destructor being called. (Speculated from source since no reliable
repro is found.)

This CL clears eveything in the destroy() function as if the destructor has
been called, only delaying memory release.

R=tommycli
BUG=483068

Review URL: https://codereview.chromium.org/1137663006

Cr-Commit-Position: refs/heads/master@{#329309}

content/renderer/pepper/pepper_plugin_instance_impl.cc

@@ -699,6 +699,8 @@ void PepperPluginInstanceImpl::Delete() {
     fullscreen_container_ = NULL;
   }
 
+  throttler_.reset();
+
   // Force-unbind any Graphics. In the case of Graphics2D, if the plugin
   // leaks the graphics 2D, it may actually get cleaned up after our
   // destruction, so we need its pointers to be up-to-date.

content/renderer/pepper/pepper_webplugin_impl.cc

@@ -124,6 +124,7 @@ void PepperWebPluginImpl::destroy() {
   // Tell |container_| to clear references to this plugin's script objects.
   if (container_)
     container_->clearScriptObjects();
+  container_ = nullptr;
 
   if (instance_.get()) {
     ppapi::PpapiGlobals::Get()->GetVarTracker()->ReleaseVar(instance_object_);
@@ -131,6 +132,7 @@ void PepperWebPluginImpl::destroy() {
     instance_->Delete();
     instance_ = NULL;
   }
+  throttler_.reset();
 
   base::MessageLoop::current()->DeleteSoon(FROM_HERE, this);
 }