Replace TaskType::kUnthrottled with kInternalDefault in BrowserPlugin

The task runner is used at UpdateInternalInstanceId and DeleteSoon, and
I think unthorttled task runner is not necessary here.

Note that the original task runner before kUnthrottled was
ThreadTaskRunnerHandle::Get().

We plan to eliminate kUnthrottled, kUnspecedTimer and kUnspecedLoading
and replace them with appropriate kInternal* task types.

Task type guideline: https://bit.ly/2vMAsQ4

Bug: 836410
Change-Id: I11679337908b6c64f3f6604639f2aed3b7c60ab0
Reviewed-on: https://chromium-review.googlesource.com/1055367Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559430}

content/renderer/browser_plugin/browser_plugin.cc

@@ -96,7 +96,8 @@ BrowserPlugin::BrowserPlugin(
       ready_(false),
       browser_plugin_instance_id_(browser_plugin::kInstanceIDNone),
       delegate_(delegate),
-      task_runner_(render_frame->GetTaskRunner(blink::TaskType::kUnthrottled)),
+      task_runner_(
+          render_frame->GetTaskRunner(blink::TaskType::kInternalDefault)),
       weak_ptr_factory_(this) {
   browser_plugin_instance_id_ =
       BrowserPluginManager::Get()->GetNextInstanceID();