Bug 219342

WebGL rendering context should track the "context has been changed" state. Currently the WebGL rendering context kind of tracks this, but still sends this also to the GraphicsContextGL(OpenGL). This state is used to understand whether the prepare for presentation should create a new frame or not. If the context has not been changed, compositing skips swapping a new frame. This should be instead tracked at the caller level and trigger compositing only when exactly needed. Having this variable complicates especially the GPU Process implementation, where this variable needs to be tracked in both web process as well as the gpu process. Having this variable complicates the WebGL rendering context implicit back buffer clear implementation, as it is consulted in redundant fashion.