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Logic to validate custom handlers is required on both the web and
browser processes. This CL introduces a new API in
third_party/blink/public/common in order to reduce duplication. As a
starting point, a new helper function allows to verify whether the
following condition is satisfied [1]:

> If scheme is neither a safelisted scheme nor a string starting with
> "web+" followed by one or more ASCII lower alphas'

In order to keep this CL small, more advanced aspects like same-origin
condition (currently performed in WebContentsImpl), validation of the
schemes of the registered URLs [2] [3] or other tests that are currently
only performed on the web process are not considered. This can be refine
later if needed.

This CL makes the check on the browser process slighty stronger.
Previously the only requirement for URLs starting with "web+" was to be
sure they are not just equal to "web+".

This CL might also make verification on the web process slightly less
efficient, if the conversion from WTF::String to base::StringPiece
requires a buffer allocation. However, it seems unlikely to be a
performance bottleneck for the current use cases.

[1] https://html.spec.whatwg.org/multipage/system-state.html#normalize-protocol-handler-parameters
[2] https://crbug.com/1112268
[3] https://crbug.com/64100

Bug: 971917, 952974
Change-Id: Iaada22200d7b7d834ad878bbc51cc40ea67d6332
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2362802
Commit-Queue: Frédéric Wang <fwang@igalia.com>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800948}