Clarify comments about the role of the lex/parse table cache action

The comment claimed the explicit cache update step wasn't required, and
explained why we still did it. This was correct for a clean build, but
for an incremental build, when stale cache files might exist, it was
incorrect; the explicit cache update step is in fact required to refresh
a stale cache.

NOTRY=true

Review URL: https://codereview.chromium.org/335053003

git-svn-id: svn://svn.chromium.org/blink/trunk@176917 bbb929c8-8fbe-4397-9dbb-9b2b20218538