Clean up extension confirmation prompts and make them consistent between Views and Cocoa.
All string changes + screenshots:
https://docs.google.com/a/google.com/spreadsheets/d/1MVv3BjTtzNHTfUbsm4_fRBOSto7aE0E03IitDmvjg8k/edit?usp=sharing
XIB changes in ExtensionInstallPromptWebstoreData.xib:
Make the title NSTextField match the one in ExtensionInstallPrompt.xib (multi-line instead of eliding).
BUG=419403
Review URL: https://codereview.chromium.org/
1056003004
Cr-Commit-Position: refs/heads/master@{#334371}