Fix refresh rate query for internal monitors

GetSelectedModeForDisplayId doesn't work for internal displays. This
results in failing the query, and falling back to refresh rate
approximation in the scheduler. To fix this, we use
GetActiveModeForDisplayId which exists even on internal displays.
This fixes the swap interval that the cc scheduler sees.

BUG=chromium:415359
TEST=make a trace on pixel, look at interval_us, it is a constant 16677 now (before it was variable and inaccurate)

Review URL: https://codereview.chromium.org/582543002

Cr-Commit-Position: refs/heads/master@{#295505}

ash/display/display_controller.cc

@@ -127,9 +127,9 @@ void SetDisplayPropertiesOnHost(AshWindowTreeHost* ash_host,
       CreateRootWindowTransformerForDisplay(host->window(), display));
   ash_host->SetRootWindowTransformer(transformer.Pass());
 
-  DisplayMode mode;
-  if (GetDisplayManager()->GetSelectedModeForDisplayId(display.id(), &mode) &&
-      mode.refresh_rate > 0.0f) {
+  DisplayMode mode =
+      GetDisplayManager()->GetActiveModeForDisplayId(display.id());
+  if (mode.refresh_rate > 0.0f) {
     host->compositor()->vsync_manager()->SetAuthoritativeVSyncInterval(
         base::TimeDelta::FromMicroseconds(
             base::Time::kMicrosecondsPerSecond / mode.refresh_rate));