Speculative mitigation for crash in DispatchTouchEvent().

From the stack traces for this bug, it looks like we're trying to access
an optional value that isn't set. While we investigate why this is
happening, let's explicitly check the value exists before accessing.

Bug: 888143
Change-Id: Ifc1e47cc26e601c450f70b07dc37e764f6543802
Reviewed-on: https://chromium-review.googlesource.com/1240129Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593629}

content/browser/renderer_host/render_widget_host_input_event_router.cc

@@ -730,9 +730,12 @@ void RenderWidgetHostInputEventRouter::DispatchTouchEvent(
     // of the touch sequence, though this could be wrong; a better approach
     // might be to always transform each point to the |touch_target_.target|
     // for the duration of the sequence.
-    DCHECK(target_location.has_value());
-    touch_target_.delta =
-        target_location.value() - touch_event.touches[0].PositionInWidget();
+    if (target_location.has_value()) {
+      touch_target_.delta =
+          target_location.value() - touch_event.touches[0].PositionInWidget();
+    } else {
+      touch_target_.delta = gfx::Vector2dF();
+    }
 
     DCHECK(touchscreen_gesture_target_map_.find(
                touch_event.unique_touch_event_id) ==