exo: Refactor ShellSurface and WaylandRemoteShell

The gfx::Size part of ShellSurface::initial_bounds_ was abused to tell
shell surface types apart, i.e. their behavior implicitly depended on
otherwise unused initial state, e.g. gfx::Size(1, 1). This CL replaces
initial_bounds_ with an origin_ and explicit state that determines how
shell surface bounds are controlled.

This CL also includes miscellaneous style cleanups.

BUG=642894
TEST=None

Review-Url: https://codereview.chromium.org/2688483003
Cr-Commit-Position: refs/heads/master@{#450533}