DevTools timeline: extract trace event bindings into a file of its own

... and decouple it from TracingModel.

BUG=361045

Review URL: https://codereview.chromium.org/296053019

git-svn-id: svn://svn.chromium.org/blink/trunk@174791 bbb929c8-8fbe-4397-9dbb-9b2b20218538
API should allow arbitrary value for Promise parameter.

As [1] says, we should convert thenables and non-promise
values to a Promise by resolving with it.

https://github.com/w3ctag/promises-guide#accepting-promises

BUG=347053

Review URL: https://codereview.chromium.org/294913002

git-svn-id: svn://svn.chromium.org/blink/trunk@174792 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Updated comment in UseCounter's CSS mapping to mention new script to
update Chrome's UMA strings.

Review URL: https://codereview.chromium.org/303483002

git-svn-id: svn://svn.chromium.org/blink/trunk@174793 bbb929c8-8fbe-4397-9dbb-9b2b20218538