Make lgtm detection have fewer false positives

Look for "lgtm" only as a whole word, so that it will not match occurances of those letters used as part of filenames or variable names in code.

I did this on the chromium branch, but I intend to commit it to the default branch too.

BUG=352637

[jrobbins (corp), 2014-08-15 19:52:27]

PTAL.  This responds to the topic of your email thread where a variable name was
detected as an "lgtm".

It was also suggested that I prefix source code with a ">" or other symbol so
that it could be easily skipped.  However, (a) the source code used as context
often word-wraps so the leading character would not be on the continued line, 
(b) often people talk about source code variables, so the user is likely to type
in alternative suggestions for code and mention the same variable names, and (c)
looking for whole words avoids those cases anyway.

[phajdan.jr, 2014-08-19 10:56:45]

LGTM

[jrobbins (corp), 2014-08-20 20:38:24]

committed as
https://code.google.com/p/rietveld/source/detail?r=414fa1f89a8df25b9c40a07865...
and a quick follow up
https://code.google.com/p/rietveld/source/detail?r=ee3b7d96ce332fe30983fe453a...