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Enable the hotseat feature as default. Besides that, this CL does the
following things:
In annotation_feature_enabled_test.js and zcr_remote_shell_unittests
.cc, the current tests assume that the shelf icon's size is 56x56 which
is not always true under hotseat. So those tests are fixed by hard code.

TBR=tbarzic@chromium.org,oshima@chromium.org
tbarzic: chromeos_features.cc
oshima: zcr_remote_shell_unittest.cc

Bug: 1032702
Change-Id: I3b51893d4724e6f085fea8ca7c0b9b415e6cea4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974861Reviewed-by: Matthew Mourgos <mmourgos@chromium.org>
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727654}