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This patch changes slightly the grammar used by the Uri class. In the
previous version, the ":" separator following the schema was always
required. In the new grammar, the ":" separator is omitted for an empty
schema. As a result, the normalized representation of an empty Uri is an
empty string. This is more intuitive behavior than the previous one,
when empty Uri was represented by ":" string. All URIs accepted by the
old grammar are also accepted by the new one. Additionally, URIs
without scheme and without the ":" separator are also accepted now.

BUG=none
TEST=with unittests on my workstation

Change-Id: I4bc8875af4a5d05a227aca47863c7d659b8d4223
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2553529
Auto-Submit: Piotr Pawliczek <pawliczek@chromium.org>
Commit-Queue: Sean Kau <skau@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831958}