Add canScroll bit to WebMouseWheelEvent

public/web/WebInputEvent.h

Index: public/web/WebInputEvent.h
diff --git a/public/web/WebInputEvent.h b/public/web/WebInputEvent.h
index ab7e4aba73b7186d8a9a70ec2efc8ebdd5da7618..172a5bd317f14cbb41d9921a18f44edacbfb3a3f 100644
--- a/public/web/WebInputEvent.h
+++ b/public/web/WebInputEvent.h
@@ -362,6 +362,13 @@ public:
     Phase momentumPhase;
 
     // See comment at the top of the file for why an int is used here.

[Rick Byers, 2014/11/27 15:55:15]

Majid is just about to land his CL using bools
(https://codereview.chromium.org/743883005/), please use a bool here as well and
remove this comment.

[lanwei, 2014/12/02 06:28:22]

I will update when I can pull the new changes from master.

[Rick Byers, 2014/12/02 15:48:20]

I talked with Majid.  His CL might still be a couple days.  We discovered that
there is already at least one bool (WebKeyboardEvent::m_isSystemKey) and that
seems to be fine.  So you can safely use 'bool' for your new property without
fear.  Majid will clean up the others.

+    // When Ctrl key is pressed, for scrolling on touch pad on ChromeOS or all
+    // scrolling on MACOS, we set suppressScroll to be false, which means always

[tdresser, 2014/11/27 15:06:06]

For consistency with other code, use "Mac OS"

[lanwei, 2014/12/02 06:28:22]

Done.

+    // scrolling, for all other scrolling, we set it true, disabling all scrolling

[tdresser, 2014/11/27 15:06:06]

I find this comment hard to read.
Can you try rewording it?

[Rick Byers, 2014/11/27 15:55:15]

How about something like this (remember blink and chromium are separate - blink
shouldn't try to dictate exactly when chromium will set this)?

When true, this wheel event should not trigger scrolling (or any other default
action) if the event goes unhandled by JavaScript. This is used, for example,
when the browser decides the default behavior for Ctrl+Wheel should be to zoom
instead of scroll.


This raises an interesting question I hadn't thought of before: should this
really be "suppressScroll" or "preventDefault"?  I'm not aware of any other
default action for wheel events in blink (other than scrolling), but if there
was one we'd probably want this bit to suppress it too.  I kind of like the
symmetry we get with Event.preventDefault by calling this 'preventDefault'. 
What do you guys think?  

[lanwei, 2014/12/02 06:28:22]

Done.

[lanwei, 2014/12/02 06:28:22]

Done.

+    // for this wheel event.
+    int suppressScroll;
+
+    // See comment at the top of the file for why an int is used here.
     // Rubberbanding is an OSX visual effect. When a user scrolls the content
     // area with a track pad, and the content area is already at its limit in
     // the direction being scrolled, the entire content area is allowed to
@@ -391,6 +398,7 @@ public:
         , hasPreciseScrollingDeltas(false)
         , phase(PhaseNone)
         , momentumPhase(PhaseNone)
+        , suppressScroll(false)
         , canRubberbandLeft(true)
         , canRubberbandRight(true)
     {