AW: use correct ClassLoader in dupeMethod

This fixes an issue with the support library where we would sometimes
create an InvocationHandler and look up methods using the wrong
ClassLoader. This ensures we always use the correct ClassLoader.

dupeMethod() is only used here, so it's safe to change the signature.

See the crbug for more information.

Bug: 826988
Test: Manual, did this in my demo CL at http://crrev/c/965883
Change-Id: I7ca7bb90aec81a6a30ce4d6393cf40ee54d40c19
Reviewed-on: https://chromium-review.googlesource.com/985631
Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#546961}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c6aed4c0fe3d8eeb3ae7f2bdd32cc6433cdf929c