b/3350774 Enforce clearing the flag mIsHandlingMultiTouch.

b/3350774 was fixed by https://android-git.corp.google.com/g/#change,91337
But there is small chance that the flag mIsHandlingMultiTouch is not cleared
correctly. This CL enforces to clear the flag.

Change-Id: I79ba6b401a7d7b3c772d3f123912343491551507

core/java/android/webkit/WebView.java

@@ -5405,6 +5405,7 @@ public class WebView extends AbsoluteLayout
             case MotionEvent.ACTION_DOWN: {
                 mPreventDefault = PREVENT_DEFAULT_NO;
                 mConfirmMove = false;
+                mIsHandlingMultiTouch = false;
                 mInitialHitTestResult = null;
                 if (!mScroller.isFinished()) {
                     // stop the current scroll animation, but if this is