DevTools: Load document (html) content from disk cache in page agent enabling.

BUG=354447
R=pfeldman@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=174557

Review URL: https://codereview.chromium.org/267393003

git-svn-id: svn://svn.chromium.org/blink/trunk@176213 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Timeline: disable start/stop button and properly indicate progress when stop is pending

Now that we may fetch a lot of trace (or buffered timeline) events after
timeline is stopped, we need a better indication of that. The button remaining
red after it's pressed is very annoying. Let's turn it off and disable till
we get all the events, and if the progress pane is on, display the appropriate
message there.

BUG=361045

Review URL: https://codereview.chromium.org/322403004

git-svn-id: svn://svn.chromium.org/blink/trunk@176214 bbb929c8-8fbe-4397-9dbb-9b2b20218538