Don't allow second finger to clobber initial press

If we are already in one finger passthrough mode
triggered by press on the edge, don't allow other
fingers to clobber the initial press. Otherwise
we will never send the release for the first
finger.

Bug: None
Test: display assistant with screen reader enabled, back gesture
Change-Id: I742102eeb3fad05f7a32b6c7cdd34b9036f6ddaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2108866
Commit-Queue: Alex Sakhartchouk <alexst@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751685}

chromecast/browser/accessibility/touch_exploration_controller.cc

@@ -195,6 +195,12 @@ ui::EventDispatchDetails TouchExplorationController::RewriteEvent(
   if (side_gesture_pass_through_ && type == ui::ET_TOUCH_PRESSED &&
       FindEdgesWithinInset(location, gesture_start_width_,
                            gesture_start_height_) != NO_EDGE) {
+    // If we are already in pass-through, ignore additional presses
+    // or the other fingers will clobber our initial press.
+    if (state_ == ONE_FINGER_PASSTHROUGH) {
+      return DiscardEvent(continuation);
+    }
+
     SET_STATE(ONE_FINGER_PASSTHROUGH);
     initial_press_ = std::make_unique<ui::TouchEvent>(touch_event);
     passthrough_offset_ = gfx::Vector2dF(0, 0);