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PLY lex table creation is (obviously) expensive.
Pre-caching this in a separate build step improves build time.

This is much less of an improvement than the parse table,
as the lexical grammar is much simpler than the phrase grammar,
but:
* it's a quick win, and
* will allow us to use Python's optimized mode (hopefully more win).

Improves build time (user time) by 5% on my Linux box:
59s => 56s

Followup to:
Pre-cache PLY yacc parse table and use optimized mode (45% build time improvement)
https://codereview.chromium.org/184233005/

BUG=341748
R=haraken

Review URL: https://codereview.chromium.org/183013013

git-svn-id: svn://svn.chromium.org/blink/trunk@168630 bbb929c8-8fbe-4397-9dbb-9b2b20218538