GitLab

Partially implement OSExchangeDataProviderMac, just enough to get
OSExchangeDataTest and TextFieldTest.DragAndDrop* to pass.
The implementation is backed by NSPasteboard, and relies on the category
NSPasteboard(ChimeraPasteboardURLUtils) defined in third_party/mozilla.

BUG=378134

Review URL: https://codereview.chromium.org/368973003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282277 0039d316-1c4b-4281-b951-d872f2087c98