[es6] emit error when for-in loop declarations are initialized in strict mode

[es6] emit error when for-in loop declarations are initialized in strict mode

The ES6 grammar forbids the initialization of variable declarations in IterationStatements.

This CL will report `for (var x = y in z)` as a SyntaxError in strict mode (as done in JSC). It is possible that this could break sites in sloppy mode, and so that change can wait.

BUG=
R=
LOG=N

Review URL: https://codereview.chromium.org/1033823002

Cr-Commit-Position: refs/heads/master@{#27639}