Add hovering feature to AshDevToolsDOMAgent

ash/common/wm_shell.cc

Index: ash/common/wm_shell.cc
diff --git a/ash/common/wm_shell.cc b/ash/common/wm_shell.cc
index 41d6b6247421cfb9d6e448affe4665db559dab27..012ac5e6800869a3660988d7740d35634fbf1b46 100644
--- a/ash/common/wm_shell.cc
+++ b/ash/common/wm_shell.cc
@@ -93,7 +93,8 @@ void WmShell::Initialize(const scoped_refptr<base::SequencedWorkerPool>& pool) {
   // Start devtools server
   devtools_server_ = ui::devtools::UiDevToolsServer::Create(nullptr);
   if (devtools_server_) {
-    auto dom_backend = base::MakeUnique<devtools::AshDevToolsDOMAgent>(this);
+    auto dom_backend = base::MakeUnique<devtools::AshDevToolsDOMAgent>(
+        this, base::MessageLoop::current()->task_runner());

[sadrul, 2016/12/06 20:48:59]

Use base::ThreadTaskRunnerHandle::Get() instead. But maybe we don't need this?
See comment in dom-agent.cc

[Sarmad Hashmi, 2016/12/06 22:54:34]

As discussed, passing main_thread_task_runner to devtools_server instead. Agents
no longer know about any task runners.

     auto css_backend =
         base::MakeUnique<devtools::AshDevToolsCSSAgent>(dom_backend.get());
     auto devtools_client = base::MakeUnique<ui::devtools::UiDevToolsClient>(