Permit MaterialDesignController to make IO call

ui/base/material_design/material_design_controller.cc

Index: ui/base/material_design/material_design_controller.cc
diff --git a/ui/base/material_design/material_design_controller.cc b/ui/base/material_design/material_design_controller.cc
index 411f553a8ba2176680caf3caebaad04334e26c84..d1b2e7cd026ced9080aae69d9fb262363a8ba41d 100644
--- a/ui/base/material_design/material_design_controller.cc
+++ b/ui/base/material_design/material_design_controller.cc
@@ -17,6 +17,7 @@
 #include <fcntl.h>
 
 #include "base/files/file_enumerator.h"
+#include "base/threading/thread_restrictions.h"
 #include "ui/events/ozone/evdev/event_device_info.h"
 #endif  // defined(USE_OZONE)
 
@@ -43,6 +44,8 @@ bool MaterialDesignController::IsModeMaterial() {
 
 MaterialDesignController::Mode MaterialDesignController::DefaultMode() {
 #if defined(OS_CHROMEOS)
+  // If a scan of available devices has already completed, use material-hybrid
+  // if a touchscreen is present.
   if (DeviceDataManager::HasInstance() &&
       DeviceDataManager::GetInstance()->device_lists_complete()) {
     return GetTouchScreensAvailability() == TouchScreensAvailability::ENABLED ?
@@ -50,6 +53,9 @@ MaterialDesignController::Mode MaterialDesignController::DefaultMode() {
   }
 
 #if defined(USE_OZONE)
+  // Otherwise perform our own scan to determine the presence of a touchscreen.
+  // Note this is a one-time call that occurs during device startup or restart.
+  base::ThreadRestrictions::ScopedAllowIO allow_io;

[sadrul, 2016/03/30 19:14:18]

I am not a fan of this. Can we just stick with the block above? How often does
the code fall back into here?

[tdanderson, 2016/03/30 19:26:12]

The block above is insufficient since whenever the UI restarts (for example,
when changing something in about:flags and then clicking restart) the async
device scan hasn't been completed by the time we reach here.

Upon booting the device, I have observed that device_lists_complete() is true
(line 50) by the time MaterialDesignController needs to know whether to use
material or material-hybrid. But I do not know enough about this to say with any
certainty that that will always be the case. (My current understanding is that
we only need to query MD vs MD hybrid after the user has logged in, and by that
time device_lists_complete() is probably going to be true)

   base::FileEnumerator file_enum(
       base::FilePath(FILE_PATH_LITERAL("/dev/input")), false,
       base::FileEnumerator::FILES, FILE_PATH_LITERAL("event*[0-9]"));