WebUI: Update Polymer iron-collapse, iron-overlay-behavior to 2.x.

Bug: 738611
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I7f3649304443c82c7604d4ba4c9e2ef0b966c5f1
Reviewed-on: https://chromium-review.googlesource.com/919643
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536919}