Timeline: merge records from different threads to they're sorted by time

Timeline records for different threads may arrive out of order. We used
to bsearch and insert them in the proper location in the presentation
model and disregard the problem in the frame model. Now we introduce
a class that buffers and merges them before they're processed.

Review URL: https://codereview.chromium.org/149623003

git-svn-id: svn://svn.chromium.org/blink/trunk@167175 bbb929c8-8fbe-4397-9dbb-9b2b20218538