Some cleanup of ParsePropertyDefinition --- Replaces certain hacks with
more structured, clean code, and adds additional comments to aid in
comprehension of this tricky area of the ambiguous recursive descent
parser.

BUG=v8:3583
LOG=N
R=adamk, aperez, wingo, rossberg

Review URL: https://codereview.chromium.org/1348773004

Cr-Commit-Position: refs/heads/master@{#30775}