(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
We didn't add buttons for certain window types or states when the
window was created, but when a window with a button changed its
state to where we would not have created the button, we didn't
remove the existing button.
Window icons in Taskbar were previously received in WM events with
shbuf ID's. Now that Gfx::ShareableBitmap is backed by anonymous files,
we can easily switch to using those.