[pinpoint] Update webapp2 version from 2.5.1 to 2.5.2.
[pinpoint] Update webapp2 version from 2.5.1 to 2.5.2.

Change-Id: Ief90811a37808f11984959b9701cf61ecfdaf9b8
Reviewed-on: https://chromium-review.googlesource.com/1225797
Reviewed-by: Simon Hatch <490a224d89645d3fea983d7189ce51bde0a00c1d@chromium.org>
Commit-Queue: Dave Tu <baa605d781958521249fdcf7bad76b01d943134a@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult