Null check secondary_pointer_down_event_

There are some crash reports in android webview that suggests this
pointer is null and used. In android webview, the app gets the touch
events first, so can modify or drop events however they want. That may
be the reason certain assumptions holds in chrome, but not in webview.

Assuming that's the case, just add null checks before use.

BUG=675772

Review-Url: https://codereview.chromium.org/2595943003
Cr-Commit-Position: refs/heads/master@{#440960}

ui/events/gesture_detection/gesture_detector.cc

@@ -311,8 +311,9 @@ bool GestureDetector::OnTouchEvent(const MotionEvent& ev) {
           if (!IsWithinTouchSlop(ev)) {
             handled = listener_->OnScroll(
                 *current_down_event_, ev,
-                (maximum_pointer_count_ > 1 ? *secondary_pointer_down_event_
-                                            : ev),
+                (maximum_pointer_count_ > 1 && secondary_pointer_down_event_)
+                    ? *secondary_pointer_down_event_
+                    : ev,
                 scroll_x, scroll_y);
             last_focus_x_ = focus_x;
             last_focus_y_ = focus_y;
@@ -329,8 +330,9 @@ bool GestureDetector::OnTouchEvent(const MotionEvent& ev) {
                    std::abs(scroll_y) > kScrollEpsilon) {
           handled = listener_->OnScroll(
               *current_down_event_, ev,
-              (maximum_pointer_count_ > 1 ? *secondary_pointer_down_event_
-                                          : ev),
+              (maximum_pointer_count_ > 1 && secondary_pointer_down_event_)
+                  ? *secondary_pointer_down_event_
+                  : ev,
               scroll_x, scroll_y);
           last_focus_x_ = focus_x;
           last_focus_y_ = focus_y;
@@ -553,7 +555,8 @@ bool GestureDetector::IsWithinTouchSlop(const MotionEvent& ev) {
     const int pointer_id = ev.GetPointerId(i);
     const MotionEvent* source_pointer_down_event = GetSourcePointerDownEvent(
         *current_down_event_,
-        maximum_pointer_count_ > 1 ? *secondary_pointer_down_event_
+        (maximum_pointer_count_ > 1 && secondary_pointer_down_event_)
+            ? *secondary_pointer_down_event_
             : *current_down_event_,
         pointer_id);
     DCHECK(source_pointer_down_event);