[Extensions UI Mac] Adjust window height calculations for the install bubble

If there isn't much content in the install bubble, the icon might have greater
height than the rest of the content. Take this into account.

BUG=570238

Review URL: https://codereview.chromium.org/1529363003

Cr-Commit-Position: refs/heads/master@{#365903}
3 files changed