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Blink's actual implementation preserves keyIdentifier='Enter'
when Enter is pressed. To make sure that the test framework
behaves like the actual implementation, this CL makes sure that
keyIdentifier is set to 'Enter' when the Enter key is simulated.

BUG=423270

Review URL: https://codereview.chromium.org/649343002

Cr-Commit-Position: refs/heads/master@{#302403}