cc: Make OutputSurfaceFrame::sub_buffer_rect optional

sub_buffer_rect is used to signal partial or empty swap, but previously
required a default value of full surface size to indicate regular
swap.  In preparation for adding swap with content bounds, clarify
the default case to not provide a value for sub_buffer_rect.

BUG=687690
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2699173002
Cr-Commit-Position: refs/heads/master@{#452234}

• cc/output/gl_renderer.cc[diff]
• cc/output/output_surface_frame.h[diff]
• cc/test/fake_output_surface.cc[diff]
• cc/test/fake_output_surface.h[diff]
• content/browser/compositor/gpu_browser_compositor_output_surface.cc[diff]
• content/browser/compositor/gpu_surfaceless_browser_compositor_output_surface.cc[diff]
• content/browser/compositor/offscreen_browser_compositor_output_surface.cc[diff]
• content/browser/renderer_host/compositor_impl_android.cc[diff]
• services/ui/surfaces/display_output_surface.cc[diff]
• services/ui/surfaces/display_output_surface_ozone.cc[diff]
• ui/compositor/test/in_process_context_factory.cc[diff]