DevTools: Smooth and faster SplitView resizing.

We should always update sidebar size in a SplitView on every onResize
event (instead of updating only the total size), otherwise on starting
a drag event the resizer position might jump "unexpectedly" to satisfy
the sidebar size constraints.

This patch also optimizes the onResize handler for the SplitView.
The bottleneck was in calculating offsetWidth/offsetHeight of the
elements. Some of those places are now cached.

BUG=245514
R=kaznacheev,apavlov

Review URL: https://chromiumcodereview.appspot.com/18465004

git-svn-id: svn://svn.chromium.org/blink/trunk@155241 bbb929c8-8fbe-4397-9dbb-9b2b20218538