[Switch Access] Enable dictation from Switch Access menu

The Switch Access menu currently contains a button to activate
dictation. However, if the user has not already enabled the feature,
the button does nothing.

This change has Switch Access enable dictation when activated.
If they used dictation previously, it enables and activates immediately.
If they have not used dictation before, it opens a confirmation dialog.

Bug: None
Change-Id: I981ba4666a89909055d27996e256d6ca8a89aa13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824318
Commit-Queue: David Tseng <dtseng@chromium.org>
Auto-Submit: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699822}

ash/accessibility/accessibility_controller_impl.cc

@@ -882,6 +882,7 @@ void AccessibilityControllerImpl::ToggleDictationFromSource(
   base::RecordAction(base::UserMetricsAction("Accel_Toggle_Dictation"));
   UserMetricsRecorder::RecordUserToggleDictation(source);
 
+  SetDictationEnabled(true);
   ToggleDictation();
 }