Introduce include directive to make_private_script_source.py

To support large Blink-In-JavaScript project, such as execCommand-in-JS, this
patch introduce "include" directive in make_private_script_source.py to bundle
multiple JavaScript files into one private script entry in C++.

Syntax of "include" directive is as same as what Chromium JavaScript resource
file used: <include src="filename">

BUG=n/a
TEST=n/a

Review URL: https://codereview.chromium.org/526023002

git-svn-id: svn://svn.chromium.org/blink/trunk@181180 bbb929c8-8fbe-4397-9dbb-9b2b20218538