DevTools: remove unused images.

R=apavlov, lushnikov, pfeldman

Review URL: https://codereview.chromium.org/145003012

git-svn-id: svn://svn.chromium.org/blink/trunk@165839 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Make event retargeting algorithms work for multiple shadow trees.

The relevant spec issue is: https://www.w3.org/Bugs/Public/show_bug.cgi?id=23769

So far, event retargeting for multiple shadow trees does't work nicely. 
Event listeners registered on nodes in the younger shadow tree can't see the
original event target in the older shadow tree.

This issue is one of the missing pieces that we have to resolve to make the
event section in the Shadow DOM spec complete.

To resolve the issue, I've updated the spec so that the current event retargeting 
algorithm will work without any modifications. What has changed is the definition 
of tree of trees. By this new definition of trees of trees, we can apply the 
current event retargeting algorithms "as-is" to achieve the desired behavior.

In the implementation, I no longer use a stack-based approach to track adjusted
targets. That's no longer an effective solution. Instead of that, I introduced a 
carefully designed recursive algorithm, which is defined in 
EventPath::ensureTreeScopeEventContext. Because we visit each ancestor TreeScope 
only once (or twice), we can still achieve O(N) (N is the number of ancestor
TreeScopes),  instead of O(N^2), to calculate adjusted targets.

BUG=337089

Review URL: https://codereview.chromium.org/145533002

git-svn-id: svn://svn.chromium.org/blink/trunk@165840 bbb929c8-8fbe-4397-9dbb-9b2b20218538