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Integrates ResizeObserver with the browser.

1) Element notifies ResizeObserver when its size might have changed.

Operations that might change Element's observed size are trapped:
Element::detachLayoutTree traps Element removal from DOM tree
LayoutObject::setMayNeedPaintInvalidation traps size changes

2) Event loop integration

FrameView::notifyResizeObserver implements it per spec.
https://wicg.github.io/ResizeObserver/#html-event-loop

3) Tests

LayoutTests/resize-observer is a comprehensive test suite.

BUG=612962

Committed: https://crrev.com/5b49641e336ba9be7789cc0eb2511d415f4e6d5d
Review-Url: https://codereview.chromium.org/2204503002
Cr-Original-Commit-Position: refs/heads/master@{#411694}
Cr-Commit-Position: refs/heads/master@{#412149}