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UserScriptSlave was responsible for too much, and the logic was getting
cluttered (which also made changes in, e.g.
https://codereview.chromium.org/288053002/ difficult to read, write, and
review). Refactor the logic to inject the scripts into a new class, so that
UserScriptSlave is pretty much just responsible for containing them.

BUG=373961

Review URL: https://codereview.chromium.org/284153006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271532 0039d316-1c4b-4281-b951-d872f2087c98