Tapping the "Show Diffs" link in the change view to display all of a
change's diffs inline spawns several asynchronous syntax highlighting
processes. Sometimes (especially for long file lists and slower HLJS
download rates) multiple highlight processes would request HLJS before
it had been loaded, resulting in multiple load/definitions of the
library. Occasionally this would result in a JS exception.
With this change, the HLJS loader functionality of GR-SYNTAX-LAYER is
separated into the GR-SYNTAX-LIB-LOADER component. This component
employs a singleton property to track state across multiple lib requests
behind a promise interface.
Tests are updated.
Change-Id: I673998d406a33afa158e04c4dbb91fe31442d527
