Fix races in CFTxtFieldUndo and CFTxtFieldRedo tests.
Fix races in CFTxtFieldUndo and CFTxtFieldRedo tests.

These tests were counting on a guarantee that value change events would come in before menu popup events. As it happens, this isn't always the case.  The tests now wait for the inital value change event to come in before performing subsequent operations, thereby avoiding the race.

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/9323025

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@120322 0039d316-1c4b-4281-b951-d872f2087c98

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