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Reason for revert: Broke Linux MSan Tests. See https://ci.chromium.org/p/chromium/builders/ci/Linux%20MSan%20Tests/21213 for first failing build.
Reland fixes uninitialized variable in shell/common/shell_content_client.h

Original change's description:
> Decouple Scheme Registration from ContentMain
>
> Split off of
> https://chromium-review.googlesource.com/c/chromium/src/+/1901591, see
> that CL (and the bug) for additional context.
>
> The followup to this CL locks the SchemeRegistry on first use to ensure
> schemes aren't added after GURL is used (which could lead to bugs). This
> means that Scheme initialization must happen before ContentMain in order
> for Java to use GURL before running ContentMain (I set the ContentClient
> in JNI_Onload, and register schemes after the CommandLine is initialized
> in content library_loader_hooks).
>
> Because of that, ContentMainDelegate had to have a ContentClient
> available earlier, so I added an API to ContentMainDelegate for the
> embedder to create their ContentClient on-demand.
>
> Because I wanted to avoid registering schemes multiple times,
> url_schemes.cc had to be made stateful and remember whether schemes had
> already been registered (Schemes could be registered by ContentMain
> after JNI_Onload already registered them for Android).
>
> Doc: https://docs.google.com/document/d/1kDKqBaq-b6EbUm0F4ea7ARoksUcj1KUx3qxFuSXEwM4/edit
>
> Bug: 783819
> Change-Id: I0f6884b2def87dcca77e722cd3d3800017c18749
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1945926
> Reviewed-by: John Abd-El-Malek <jam@chromium.org>
> Reviewed-by: Albert J. Wong <ajwong@chromium.org>
> Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#725799}

TBR=ajwong@chromium.org,mthiesse@chromium.org,jam@chromium.org

Change-Id: I27a2a49dc23610350ec9f3f96ed9824e2eeed0d5
Bug: 783819
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1973198
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#725981}