Add logic to convert empty SiteInstances to a default SiteInstance.

This change allows a top-level empty SiteInstance to become the default
SiteInstance if the URL for the top-level frame does not require a
dedicated process. This allows the top-level frame and any subframes
that do not require a dedicated process to share the same default
SiteInstance process.

Bug: 958060,78757
Change-Id: I920860b822474157897a0f3e3534cca87984599c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693778
Commit-Queue: Aaron Colwell <acolwell@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680472}
18 files changed