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This creates chromium.web.CreateContextParams2 and
chromium.web.LoadUrlParams2 as FIDL tables, which are extensible. All
the call sites for the previous APIs are also updated. The original
APIs will be removed in a future CL when all out-of-tree callers for
them will have been updated.

Bug: 931831
Change-Id: I15ba10012734daa57cdb6034bbaade1253715e0c
Reviewed-on: https://chromium-review.googlesource.com/c/1476184
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634435}