Fix a cause of flakyness in NativeTextfieldViewsTest.ReadOnlyTest.

Fix a cause of flakyness in NativeTextfieldViewsTest.ReadOnlyTest.

Make the test set the clipboard first before testing
that CUT doesn't set the clipboard. Without this, the
test would fail when run a second time due to existing
clipboard contents.

Also, refactor the code to add test helper methods for
getting and setting clipboard text.

BUG=none
TEST=Run NativeTextfieldViewsTest.ReadOnlyTest twice. The second time should not fail.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=110118

[sky, 2011-11-15 16:55:19]

LGTM

[commit-bot: I haz the power, 2011-11-15 17:06:07]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asvitkine@chromium.org/8570002/1

[commit-bot: I haz the power, 2011-11-15 18:22:44]

Change committed as 110118