Timeline: add layer details view

This brings layer details view from Layers panel to Timeline's layer
tree view. Also:

- fixed trace layer import to include compositing reasons from trace;
- show compositing traces as a list of user-friendly (long) strings.

BUG=

Review URL: https://codereview.chromium.org/670503002

git-svn-id: svn://svn.chromium.org/blink/trunk@184010 bbb929c8-8fbe-4397-9dbb-9b2b20218538