Bump Chrome's deployment target to macOS 10.8.

I also updated the comment to better reflect the relationship between Chrome and
its dependencies. Several Chrome dependencies require a different min sdk and/or
deployment target from Chrome.

BUG=622481

Committed: https://crrev.com/3dade40fe88881864a42134f92238a6d292d8505
Review-Url: https://chromiumcodereview.appspot.com/2420233002
Cr-Original-Commit-Position: refs/heads/master@{#425870}
Cr-Commit-Position: refs/heads/master@{#426822}
1 file changed