Call IsolateAllSitesForTesting earlier in test setup.

Calling IsolateAllSitesForTesting() in a test body may lead to
inconsistent results for test expectations that rely on
AreOriginKeyedProcessesEnabledByDefault, since that allows the
main frame to be created with an incorrect default isolation state.
This CL fixes NavigationRequestBrowserTest.StartToCommitMetrics to
call IsolateAllSitesForTesting() in SetUpCommandLine instead to
avoid this problem.

Bug: 40259221
Change-Id: I022252d0455f717dee3f62a73de88552bcd3a4a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5716898
Commit-Queue: James Maclean <wjmaclean@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1329056}