Implement suspend() and resume() for OfflineAudioContext

LayoutTests/webaudio/audiocontext-suspend-resume-expected.txt

-Test suspend/resume for an (offline) AudioContext
+Test suspend/resume for an AudioContext
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
-PASS offlineContext = new OfflineAudioContext(1, durationInSeconds * sampleRate, sampleRate) did not throw exception.
-PASS offlineContext.state is "suspended"
-PASS p1 = offlineContext.suspend() did not throw exception.
-PASS p1 is an instance of Promise
-PASS offlineContext.suspend() was correctly rejected: InvalidAccessError: cannot suspend an OfflineAudioContext
-PASS p2 = offlineContext.resume() did not throw exception.
-PASS p2 is an instance of Promise
-PASS offlineContext.state is "suspended"
-PASS offlineContext.resume() was correctly rejected: InvalidAccessError: cannot resume an OfflineAudioContext
-PASS p3 = offlineContext.startRendering() did not throw exception.
-PASS offlineContext.state is "closed"
-PASS offlineContext.suspend() on a closed context rejected: InvalidAccessError: cannot suspend an OfflineAudioContext
-PASS offlineContext.resume() on a closed context rejected: InvalidAccessError: cannot resume an OfflineAudioContext
+PASS typeof context.suspend() is equal to function.
+PASS typeof context.resume() is equal to function.
+PASS context.suspend() resolved correctly.
+PASS Calling multiple context.suspend() did not throw an exception.
+PASS Calling context.suspend() after close() rejected correctly (with InvalidStateError: Cannot suspend a context that has been closed).
+PASS Calling context.resume() after close() rejected correctly (with InvalidAccessError: cannot resume a closed AudioContext).

[Raymond Toy, 2015/06/16 17:57:09]

We should probably make the error messages in line 10 and 11 more consistent.
(Not in this CL, though.)

[hongchan, 2015/06/16 21:31:20]

Acknowledged.

 PASS successfullyParsed is true
 
 TEST COMPLETE