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Consolidates the forked OnLoadScriptInjectors into one single shared
implementation. The shared implementation is a necessary prerequisite
step for developing platform-agnostic bindings browsertests.

* Deletes Fuchsia and Cast specific OnLoadScriptInjectors.
* Moves most browser process logic into a new class
  "OnLoadScriptInjectorHost", to further harmonize the platforms.
* Migrate Mojo interface from ScopedSharedBufferHandle to ReadOnlySharedMemoryRegion.

Merge-With: eureka-internal/432742
Bug: 1113289
Bug: 1060846
Bug: 1103058
Test: cast_shell_browsertests and web_engine_browsertests both pass.
Change-Id: If9fd2c54de08d9bc225771b01cdb32152a95fde8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2286193
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Sharon Yang <yangsharon@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795103}