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This is in preparation of migrating to Polymer3.

Specifically:
 - Adjust icon color in dark mode, previously same color was used
   in both light and dark mode.
 - Correctly hide "Learn more" link when it should be hidden.
   Previously was not correctly hidden because :empty CSS selector
   was always false.
 - Add missing icon on 3rd row of profile-blocking-error-message.
 - Remove unnecessary white space between #normal-error-message and
   #action-container.

In the process of fixing these bugs and paving the way for Polymer3:
 - Stop using "flattenhtml" to embed icons, instead use same icons
   from shared cr_elements/icons.html along with iron-icon.html
 - Extract most UI and logic in a <signin-error-app> element.
 - Leverage Polymer bindings to determine what to show/hide instead of
   :empty CSS selectors.

Bug: 1012533
Change-Id: Iaed8fdabbeb821f5c4d5b52f76510d477c55fd1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2130809Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755510}