Non-functional OSK cleanups in CompositorViewHolder

Refactors to cleanup WebContents sizing code in CompositorViewHolder:

 * Rename setSize to updateWebContentsSize to better describe what it
   does.
 * Pass in Tab rather than WebContents since callers always get it from
   the Tab.
 * Similarly, width and height always come from getViewportSize
   so inline that as well.
 * Since size is no longer passed in, unit tests must now mock the view
   size, more general readability improvements in tests.
 * Inline setSizeOfUnattachedView into updateWebContentsSize since
   that's the only callsite.
 * Inline oskResizesVisualViewport at callsites.

Change-Id: If2c8df786bed72d1974b39cfc1c0fc08a633a22d
Bug: 1384433
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4289693
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1114031}