DevTools: minimize style recalc & layout thrashing upon resize of layers 3d view.

- replace content of <style>'s only text child, not textContent, to workaround for
    duplicate style recalculation upon assignment to StyleElement.textContent (one for removeChildren(),
    another for appendChild() done from within setTextContent()).
- cache client width/height in _update();
- merge scale calculation/application;
BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/23449007

git-svn-id: svn://svn.chromium.org/blink/trunk@157213 bbb929c8-8fbe-4397-9dbb-9b2b20218538