Do not calculate composition bounds until IME requests.

Calculating composition bounds is not a light-weight operation.
On the other hand, few IME uses this information.
To skip this unnecessary calculation on certain environment, propagate
cursor update mode from browser to the renderer.

BUG=624714

Committed: https://crrev.com/dac0c7af14c29c9d8509b775d5044e5448b6263b
Review-Url: https://codereview.chromium.org/2121953002
Cr-Original-Commit-Position: refs/heads/master@{#408900}
Cr-Commit-Position: refs/heads/master@{#410927}
11 files changed