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This CL reworks the 'default browser' messages displayed in different
situations. The error icon has been removed to match the specs. A browser proxy
and unit tests have been added.

BUG=623328
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2317693003
Cr-Commit-Position: refs/heads/master@{#417327}