[Home] Check motion events before processing them

If the user is scrolling on the bottom toolbar before the browser is
ready and continues through the ready state, the gesture event filter
starts processing the stream without a down event, causing a crash.
This change adds null checks for the initial event in the gesture
detector to defend against this.

BUG=786889

Change-Id: I68a10c64653957f659e642f089a9037fddcbee5d
Reviewed-on: https://chromium-review.googlesource.com/779866Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518323}

chrome/android/java/src/org/chromium/chrome/browser/widget/bottomsheet/BottomSheet.java

@@ -366,11 +366,14 @@ public class BottomSheet
     private class BottomSheetSwipeDetector extends GestureDetector.SimpleOnGestureListener {
         @Override
         public boolean onDown(MotionEvent e) {
+            if (e == null) return false;
             return shouldGestureMoveSheet(e, e);
         }
 
         @Override
         public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
+            if (e1 == null) return false;
+
             if (!canMoveSheet()) {
                 // Currently it's possible to enter the tab switcher after an onScroll() event has
                 // began. If that happens, reset the sheet offset and return false to end the scroll
@@ -426,7 +429,7 @@ public class BottomSheet
 
         @Override
         public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
-            if (!shouldGestureMoveSheet(e1, e2) || !mIsScrolling) return false;
+            if (e1 == null || !shouldGestureMoveSheet(e1, e2) || !mIsScrolling) return false;
 
             cancelAnimation();