Make DOMWrapperWorld::current() callable from workers

This CL does the following two things:

- Makes DOMWrapperWorld::current() callable from workers. Now that a world is always stored in PerContextDataHolder::world(), DOMWrapperWorld::current() can just look up the PerContextDataHolder::world().

- Makes DOMWrapperWorld::current() callable even when a window object is being initialized. It's possible that some methods call DOMWrapperWorld::current() while the window is being initialized. To handles the subtle situation, this CL adds DOMWrapperWorld::worldOfInitializingWindow.


BUG=341032
NOTRY=true

Review URL: https://codereview.chromium.org/182903003

git-svn-id: svn://svn.chromium.org/blink/trunk@168880 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Unused 'OwnPtr' hash traits

This patch removes the unused members and types from
'HashTraits<OwnPtr<P> >'. At the moment we do not
allow 'OwnPtr' hashing, moreover some of the removed
types are not compatible with the current implementation
of 'HashTableIterator'.

Review URL: https://codereview.chromium.org/188733002

git-svn-id: svn://svn.chromium.org/blink/trunk@168881 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove histogram in ShadowRoot constructor

This histogram takes too much time. If we want to track these stats, we can use
a UseCounter instead.

R=esprehn@chromium.org
BUG=351135

Review URL: https://codereview.chromium.org/193683002

git-svn-id: svn://svn.chromium.org/blink/trunk@168882 bbb929c8-8fbe-4397-9dbb-9b2b20218538