Introduce TreeScopeEventContext

Introduce TreeScopeEventContext.

Some information can be shared between EventContexts whose TreeScope is the same.
Instead of storing such information in each EventContext (which is created per Node), store such information in newly introduced TreeScopeEventContext (which is created per TreeScope).

Though it might take some computation cost to share such information, the pros is much bigger than the cons.
We have to traverse only treeScopes, rather than nodes, when calculating some information, such as adjusted relatedTarget, adjusted event path, adjusted touch targets and so on.

We can get benefits both in time and space as follows:

1. Time
The result of the benchmarks are:

PerformanceTests/Events/EventDispatching.html
541 runs/s vs 594 runs/s  (10% better score)
    PerformanceTests/Events/EventDispatchingInShadowTrees.html
4625 runs/s vs 5741 runs/s (24% better score)

2. Space
We can reduce the memory footprint about 1.5 KBytes per an event.

BUG=329445
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=164501

[hayato, 2013-12-24 08:52:36]

PTAL.

I am not in a hurry. :)
Enjoy your vacation.

[dglazkov, 2013-12-26 17:30:13]

LGTM, this looks great.

https://codereview.chromium.org/108723007/diff/1/Source/core/events/EventCont...
File Source/core/events/EventContext.h (right):

https://codereview.chromium.org/108723007/diff/1/Source/core/events/EventCont...
Source/core/events/EventContext.h:58: class SharedEventContext : public
RefCounted<SharedEventContext> {
"Shared" is a bit too vague here. To be fair, "EventContext" is also a bit too
vague, and that is totally my fault.

Maybe TreeEventContext and NodeEventContext?

[hayato, 2014-01-06 07:33:47]

Thank you for the review.

On 2013/12/26 17:30:13, Dimitri Glazkov wrote:
> LGTM, this looks great.
> 
>
https://codereview.chromium.org/108723007/diff/1/Source/core/events/EventCont...
> File Source/core/events/EventContext.h (right):
> 
>
https://codereview.chromium.org/108723007/diff/1/Source/core/events/EventCont...
> Source/core/events/EventContext.h:58: class SharedEventContext : public
> RefCounted<SharedEventContext> {
> "Shared" is a bit too vague here. To be fair, "EventContext" is also a bit too
> vague, and that is totally my fault.
> 
> Maybe TreeEventContext and NodeEventContext?

Okay. Let me rename SharedEventContext to TreeScopeEventContext. I think it
might be better to use 'TreeScope' instead of 'Tree' here to make its meaning
clear.

As for EventContext, I'll rename it in another patch to reduce the size of this
patch.

[commit-bot: I haz the power, 2014-01-06 08:30:35]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/hayato@chromium.org/108723007/140001

[commit-bot: I haz the power, 2014-01-06 10:13:07]

Change committed as 164501

[hayato, 2014-01-06 10:51:15]

On 2014/01/06 10:13:07, I haz the power (commit-bot) wrote:
> Change committed as 164501

It seems that I forgot to rename some member variables and parameter names.
Let me do that in a follow-up patch.