ExtensionInstallDialogView: fix scrolling behavior on Views (Win,Linux)
Before:
Everything in the Views dialog (including title and icon) is in a ScrollView. This is different from the Cocoa version, in which only the content (permissions list etc) scrolls.
Also, the scrollbar overlaps the "close" button, see crbug.com/508040.
After:
Only the content scrolls. Title and icon are pulled out of the ScrollView.
Followup to https://codereview.chromium.org/
1056003004
BUG=419403,508040
Review URL: https://codereview.chromium.org/
1237483005
Cr-Commit-Position: refs/heads/master@{#338861}