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1) We need the outer-container to be scrollable when gaia screen and
virtual keyboard are shown together, so the dark overlay ('login-shield')
should always have the same size with the larger one of
the outer-container and the scroll-container, otherwise we see areas
without overlay when scrolling up.

2) We should remember to clear contents on the small pods container
before rebuilding pods.

BUG=718156
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2916973002
Cr-Commit-Position: refs/heads/master@{#476497}