First JavaScript testing implementation.

This is a plan for testing JS inside of pdf files under pdfium:

Communication of results will be done via app.alert(), rather than
console.println(), since the latter is not hooked up to any API
callbacks.

pdfium_test.cc is modified to be more careful about use of stdout/stderr,
so that only the app.alert() and the unsupported feature callback use
stdout.  Diffing stdout against ..._expected.txt files gives the result.

I added a /javascript directory to separate these from the embeddertest
resources.

The alert callback is backported from XFA.  This was an omission.

BUG=https://code.google.com/p/pdfium/issues/detail?id=62
R=jam@chromium.org, thestig@chromium.org

Review URL: https://codereview.chromium.org/872103003