Bug 882535 - Set HTMLTrackElement Readiness State during WebVTT Parsing. r=rillian

This patch enables "LOADING", "LOADED" and "ERROR" readiness states to be
applied to the Parent HTMLTrackElement, in order to avoid test flakiness,
and to comply with the HTML5 draft
http://www.w3.org/html/wg/drafts/html/master/embedded-content-0.html#text-track-readiness-state.
Bug 880676 - Correct the composition bounds to be in screen coordinates rather than layer coordinates. r=kentuckyfriedtakahe
Bug 880676 - Convert another gfx::Point to a ScreenPoint. r=BenWa
Bug 880676 - Convert some things in APZC::SampleContentTransformForFrame. r=kentuckyfriedtakahe
Bug 880676 - Make the ViewTransform classes store the unscaled layer translation. r=BenWa
Bug 880676 - Units.h cleanup for easier reading of future patches. r=BenWa
Bug 880676 - Move the RoundToInt functions to be standalone in the gfx headers. r=BenWa
Bug 880676 - Introduce a ScaleFactor struct and use it to type conversions between units. r=BenWa,kentuckyfriedtakahe
Bug 880676 - Replace a couple of Rect scaling functions with strongly typed operators. r=BenWa
Bug 880676 - Convert WidgetSpaceToCompensatedViewportSpace to the new gfx types. r=kentuckyfriedtakahe
Bug 880676 - Remove ToCSSIntRectRoundIn and convert APZC::CalculateResolution to return a ScaleFactor. r=kentuckyfriedtakahe
Bug 878901: Create a separate client front object, RootClient, for communicating with root actors. r=dcamp
Bug 878958: Use arrow functions in JS debugger client code. r=dcamp
Bug 878958: Use a Map to map actor names to reply handlers, not an object. r=dcamp
Bug 878958: In DebuggerClient, substitute mainRoot for _connected. r=dcamp
Bug 878958: Separate out a function for setting reply handlers. r=dcamp
Bug 878958: Handle a greeting packet as if it were reply from the root actor. r=dcamp