Bug 1461746 - 5. Return null from getGlobalDebugObject for promises.js object grips; r=nchevobbe

GeckoView Java tests will be using the promises actor for waiting on JS
promises to settle during a test. However, when examining object grips
that the promises actor returns, devtools often throws an exception due
to getGlobalDebugObject being called on the grip [1]. This patch makes
it return null to avoid the exception.

[1] https://dxr.mozilla.org/mozilla-central/rev/45ec8fd380dd2c308e79dbb396ca87f2ce9b3f9c/devtools/server/actors/promises.js#129

MozReview-Commit-ID: C1nFgNdQn14

--HG--
extra : rebase_source : b854ec32dea179eeafea3979e026ca3dc81754d3

devtools/server/actors/promises.js

@@ -126,8 +126,7 @@ var PromisesActor = protocol.ActorClassWithSpec(promisesSpec, {
       sources: () => this.parentActor.sources,
       createEnvironmentActor: () => DevToolsUtils.reportException(
         "PromisesActor", Error("createEnvironmentActor not yet implemented")),
-      getGlobalDebugObject: () => DevToolsUtils.reportException(
-        "PromisesActor", Error("getGlobalDebugObject not yet implemented")),
+      getGlobalDebugObject: () => null,
     });
 
     this._navigationLifetimePool.addActor(actor);