identifier. Delete unused (and now incorrect) function IsValidStrictVariable.

Implements the strong mode proposal's static restrictions on the use of the
identifier 'undefined'. Assumes these restrictions are intended to be identical
to the restrictions on the use of 'eval' and 'arguments' in strict mode. The
AllowEvalOrArgumentsAsIdentifier enum has been renamed to
AllowRestrictedIdentifiers as logic involving it is now also used for this case.

BUG=v8:3956

LOG=N

Review URL: https://codereview.chromium.org/1070633002

Cr-Commit-Position: refs/heads/master@{#27744}