Skip on-demand postMessage proxy creation for same-FrameTree messages.

When a cross-process postMessage is sent between frames in the same
FrameTree, there is no need to run the on-demand proxy creation code
in CreateOpenerProxies, because all frames within the same FrameTree
should already have proxies to one another, as they can always walk
the frame tree to reference one another. So, we can skip
CreateOpenerProxies in this case. This is a performance optimization,
since there've been reports that CreateOpenerProxies may take a
non-trivial amount of time for complex frame trees. The remaining
cases where proxies should be created would involve window.open() and
openers, which are much less common.

This change should bring down the cross-process postMessage processing
time measured by SiteIsolation.CrossProcessPostMessageTime, and also
cut down the percentage of cases that create on-demand proxies, measured
by SiteIsolation.CrossProcessPostMessage.CreateOpenerProxiesCalled which
is added in this CL.

Bug: 339512240, 41412256
Change-Id: Ib56d405bdcb5bb963798f139c976c074b584bfde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6005048
Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: Chris Thompson <cthomp@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1382439}