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This CL makes the start page fully fill the app list bounds. This will
ensure the start page WebContents has maximal space. In order to do this,
this CL decouples code from some layout assumptions and uses more
future-proof calculations instead.

BUG=453205

Review URL: https://codereview.chromium.org/889403005

Cr-Commit-Position: refs/heads/master@{#315273}