[Fuchsia] Support embedder-provided UserAgent product & version.

Bug: 969252
Change-Id: Ia54129af9dbcb727aab1d3289be997a31143e004
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649186
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Thiemo Nagel <tnagel@chromium.org>
Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672700}

• fuchsia/base/frame_test_util.cc[diff]
• fuchsia/base/frame_test_util.h[diff]
• fuchsia/engine/BUILD.gn[diff]
• fuchsia/engine/browser/web_engine_content_browser_client.cc[diff]
• fuchsia/engine/common.cc[diff]
• fuchsia/engine/common.h[diff]
• fuchsia/engine/context_provider_impl.cc[diff]
• fuchsia/engine/integration_tests_sandbox_policy[diff]
• fuchsia/engine/web_engine_integration_test.cc[Added - diff]
• fuchsia/runners/cast/cast_runner_integration_test.cc[diff]