Moved constants out of web-view.coffee and into a new file

Imported from:
https://chromium.googlesource.com/chromium/src/+/ccbfe27c8d2b4fcb7ddc98064ec6aa5820fcfe9a
Webview attributes overhaul

Imported from:
https://chromium.googlesource.com/chromium/src/+/884a5b43cba6a19b8d067fed99dc2906ed0ef656