SideBySide2: Defer syntax highlighting on large files

If a file contains more than 500 lines on either side defer syntax
highlighting by 250 milliseconds. This gives the browser a chance
to get the file opened and visible on screen before the active mode
starts chugging through the text building up the DOM.

Users see the file appear, and then CM3 highlights the visible
region and colors fill in slightly later. This gives the UI a
slightly more responsive feeling while still offering users the
syntax highlighting they want visible.

Change-Id: I86307406b93690d864e9fa652b19157667a75a82
Merge "SideBySide2: Defer syntax highlighting on large files"