js2c.py now distinguishes between the JS internal sources and any
extra scripts passed in. The latter only get validation and
comment/trailing whitespace removal applied now.

Also added a --js option to js2c.py, which will output a JS file
instead of a C++ file, for debugging. (I got tired of finding the
generated .cc file and extracting its byte array so that I could feed
it to a separate helper script I wrote.)

R=yangguo@chromium.org, jochen@chromium.org
BUG=v8:4064
LOG=N

Review URL: https://codereview.chromium.org/1117663002

Cr-Commit-Position: refs/heads/master@{#28133}