[chromecast] Disengage magnifier when screenreader enabled.

Make it so any active magnification is turned off when the user turns
on the screenreader. Because:

  - The triple tap gesture to enable/disable magnification does not
    work while screenreader is active.

  - The management of the two finger gestures inside the magnifier
    conflict with the touch exploration controller.

Bug: internal b/112634029
Test: manual on device
Change-Id: I26f39ad4bc82725f19dbd7e1caab97808eeb6de4
Reviewed-on: https://chromium-review.googlesource.com/1177636
Commit-Queue: Ryan Daum <rdaum@chromium.org>
Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583758}

chromecast/browser/accessibility/accessibility_manager.cc

@@ -75,6 +75,16 @@ void AccessibilityManager::HideHighlights() {
 
 void AccessibilityManager::SetScreenReader(bool enable) {
   touch_exploration_manager_->Enable(enable);
+
+  // TODO(rdaum): Until we can fix triple-tap and two finger gesture conflicts
+  // between TouchExplorationController, FullscreenMagnifier, and
+  // TripleTapDetector, we have to make sure magnification is not on while
+  // screenreader is active.
+  // The triple-tap gesture can still be enabled, but will not do anything until
+  // screenreader is disabled again.
+  if (enable) {
+    magnification_controller_->SetEnabled(false);
+  }
 }
 
 void AccessibilityManager::SetTouchAccessibilityAnchorPoint(