DevTools: [SSP] introduce promises for fetching style cascades

This patch introduces promises for fetching style cascades. Every
promise gets cached for future reuse; the cache gets reset whenever
the model gets changed.

In future, all these cascade fetching methods will be extracted into a
separate model (named NodeCascadeFetcher) which will be shared between
StylesSidebarPane and ComputedStylesSidebarPane. An approximate look
and feel we're striving to get to is shown here: https://crrev.com/768063003

The patch also rewrites rebuildUpdate method to use these promises.

R=vsevik

Review URL: https://codereview.chromium.org/818943003

git-svn-id: svn://svn.chromium.org/blink/trunk@187655 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: Fix navigation to breakpoint location on click in the Breakpoints pane

R=vsevik

Review URL: https://codereview.chromium.org/821973002

git-svn-id: svn://svn.chromium.org/blink/trunk@187660 bbb929c8-8fbe-4397-9dbb-9b2b20218538