Give workers their own ScriptChan and use it for postMessage.

This ensures that XHR callbacks for XHR objects in workers are called on the
worker thread rather than the main thread.
Improve the documentation for ScriptMsg.
Merge pull request #3071 from Ms2ger/workers-threads

Give workers their own ScriptChan and use it for postMessage; r=Manishearth
Change line-height to be read from the font itself, rather than
a hard-coded estimate.
Merge pull request #3074 from glennw/font-line-height

Change line-height to be read from the font itself, rather than a hard-coded estimate.
Add ahem test font for ref tests to use. Add ref test for line-height using ahem as web font.
Make libscript compilation more verbose on Travis.
Merge pull request #3072 from Ms2ger/libscript-log-travis

Make libscript compilation more verbose on Travis; r=Manishearth+SimonSapin
Merge pull request #3075 from glennw/add-ahem-font

Add ahem test font for ref tests to use. Add ref test for line-height using ahem as web font.
Make pushing to GitHub Pages verbose.
Pass the script channel to DedicatedWorkerGlobalScope::run_worker_scope rather than creating it there.

This allows us to create the Worker object before calling
DedicatedWorkerGlobalScope::run_worker_scope, which is necessary to pass a
pointer to the worker to it.
Store a pointer to the Worker in the DedicatedWorkerGlobalScope.
Implement DedicatedWorkerGlobalScope.postMessage.
Distinguish the sender to the own thread and to the parent thread in DedicatedWorkerGlobalScope.

The WorkerGlobalScope stores a reference to the sender for the own thread, to
allow passing clones to sub-workers. The DedicatedWorkerGlobalScope
additionally keeps a reference to the sender for the (unique) parent thread,
to implement postMessage and memory management of the Worker object.
Merge pull request #3076 from Ms2ger/DWGS-postMessage

Implement DedicatedWorkerGlobalScope.postMessage; r=Manishearth
Implement {Worker,DedicatedWorkerGlobalScope}.onmessage.
Merge pull request #3077 from Ms2ger/onmessage

Implement {Worker,DedicatedWorkerGlobalScope}.onmessage; r=abinader+Manishearth
Enable some workers tests.
Disable Worker_terminate_event_queue.htm for its thousands of lines of output.
Update expected results in non-debug builds.