ResizeObserver pt3: observation computation

This patch fills in ResizeObservation functionality:
- computing/setting observation size
- determining target depth

ResizeObservation::getTargetSize is static because it will also
get used by ResizeObserverEntry

I've also included basic C++ tests.

BUG=612962

Review-Url: https://codereview.chromium.org/2173203002
Cr-Commit-Position: refs/heads/master@{#407736}
5 files changed