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Undo and redo are mapped in a similar way to cut/copy/paste on Mac, via
the mainMenu's performKeyEquivalent, and the Cocoa responder chain.

This CL adds responders for undo: and redo:

Single-character editing commands were not being merged into a single
edit. For example, typing {'c', 'a', 't', Cmd+Z} would only undo the 't'
not the entire 'cat'. This CL fixes by detecting single-character
inserts in response to a keyDown event.

Then, for "redo", some of the textfield unittests would send both Ctrl+Y
and Ctrl+Shift+Z. Cmd+Y for undo isn't supported on Mac (it's mapped to
History -> Show Full History). So a wrapper is added that just sends
Cmd+Shfit+Z again.

Gets the following tests passing:
     TextfieldTest.DragAndDrop_ToTheLeft
     TextfieldTest.DragAndDrop_ToTheRight
     TextfieldTest.UndoRedoTest

BUG=454353

Review URL: https://codereview.chromium.org/923903002

Cr-Commit-Position: refs/heads/master@{#316992}