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Currently, URLs calculated by protocol handlers escape spaces as "+".
This CL changes this to use "%20" instead, aligning with Firefox and
the specification.

For details, read the following links:
https://chromestatus.com/feature/5678518908223488
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/osabCTBhDSs
https://bugs.chromium.org/p/chromium/issues/detail?id=795919
https://html.spec.whatwg.org/multipage/system-state.html#custom-handlers
https://github.com/whatwg/html/issues/3377

Bug: 1110842
Change-Id: I31e2250c56d3c4654d5a8655ead953af33b5fb5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2324126Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Frédéric Wang <fwang@igalia.com>
Cr-Commit-Position: refs/heads/master@{#799541}