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The ExtensionFunction test delegate basically just served as a way of
getting the results from the function, but we already have that by simply
setting the response callback in the function. Remove the
DelegateForTests class and update uses of it to use the callback.

Also add a response() field on ExtensionFunction for use in determining
if the function succeeded or failed. Previously, code would rely on
whether or not results_ were set, but this was wrong for a few reasons:
- The absence of results doesn't always indicate failure
- The presence of results doesn't always indicate success
- In practice, we always send back an empty list value (even if
  results weren't previously set).
Update code to use the response() value instead.

BUG=648276
TBR=benwells@chromium.org (apps browsertest change)

Review-Url: https://codereview.chromium.org/2348723002
Cr-Commit-Position: refs/heads/master@{#419560}