PPAPI: Don't use MessageLoop::current in the plugin process

The plugin is allowed to use Pepper without having bound a PPB_MessageLoop
to the calling thread, which means MessageLoop::current won't be valid.
Instead, we should just post to the Pepper main thread.

BUG=459671

Review URL: https://codereview.chromium.org/938843002

Cr-Commit-Position: refs/heads/master@{#316924}

ppapi/proxy/ppb_image_data_proxy.cc

@@ -282,7 +282,7 @@ void ImageDataCache::Add(ImageData* image_data) {
   cache_[image_data->pp_instance()].Add(image_data);
 
   // Schedule a timer to invalidate this entry.
-  base::MessageLoop::current()->PostDelayedTask(
+  PpapiGlobals::Get()->GetMainThreadMessageLoop()->PostDelayedTask(
       FROM_HERE,
       RunWhileLocked(base::Bind(&ImageDataCache::OnTimer,
                                 weak_factory_.GetWeakPtr(),

ppapi/proxy/ppb_instance_proxy.cc

@@ -930,7 +930,7 @@ void PPB_Instance_Proxy::SelectionChanged(PP_Instance instance) {
   data->should_do_request_surrounding_text = true;
 
   if (!data->is_request_surrounding_text_pending) {
-    base::MessageLoop::current()->PostTask(
+    PpapiGlobals::Get()->GetMainThreadMessageLoop()->PostTask(
         FROM_HERE,
         RunWhileLocked(base::Bind(&RequestSurroundingText, instance)));
     data->is_request_surrounding_text_pending = true;

ppapi/proxy/ppb_var_deprecated_proxy.cc

@@ -22,6 +22,7 @@
 #include "ppapi/proxy/ppp_class_proxy.h"
 #include "ppapi/proxy/proxy_object_var.h"
 #include "ppapi/proxy/serialized_var.h"
+#include "ppapi/shared_impl/ppapi_globals.h"
 #include "ppapi/shared_impl/ppb_var_shared.h"
 #include "ppapi/shared_impl/proxy_lock.h"
 #include "ppapi/shared_impl/var.h"
@@ -379,7 +380,7 @@ void PPB_Var_Deprecated_Proxy::OnMsgReleaseObject(int64 object_id) {
   // spurious warning).
   // TODO(piman): See if we can fix the IPC code to enforce strict ordering, and
   // then remove this.
-  base::MessageLoop::current()->PostNonNestableTask(
+  PpapiGlobals::Get()->GetMainThreadMessageLoop()->PostNonNestableTask(
       FROM_HERE,
       RunWhileLocked(base::Bind(&PPB_Var_Deprecated_Proxy::DoReleaseObject,
                                 task_factory_.GetWeakPtr(),

ppapi/shared_impl/resource_tracker.cc

@@ -90,7 +90,7 @@ void ResourceTracker::ReleaseResource(PP_Resource res) {
 }
 
 void ResourceTracker::ReleaseResourceSoon(PP_Resource res) {
-  base::MessageLoop::current()->PostNonNestableTask(
+  PpapiGlobals::Get()->GetMainThreadMessageLoop()->PostNonNestableTask(
       FROM_HERE,
       RunWhileLocked(base::Bind(&ResourceTracker::ReleaseResource,
                                 weak_ptr_factory_.GetWeakPtr(),