DevTools: Fix regression that slows down "Goto Source" dialog

The regression was caused by r174810. The root cause is the
implementation of WebInspector.StaticViewportElement which doesn't
support lazy DOM element creation. So every time the viewport processed
cumulative heights of inner elements, it actually forced all DOM elements
for viewport to be created.

To keep things simple, the patch moves fastHeight calls out of
ViewportElement interface into ViewportProvider.

BUG=377749
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/305813005

git-svn-id: svn://svn.chromium.org/blink/trunk@175059 bbb929c8-8fbe-4397-9dbb-9b2b20218538