Remove WaitForRenderFrameReady usage from site_per_process_browsertest.cc.
Remove WaitForRenderFrameReady usage from site_per_process_browsertest.cc.

NavigateFrameToURL was changed to wait for LoadStopped event, which
obsoletes the usage of WaitForRenderFrameReady. This CL removes it from
site_per_process_browsertest.cc.

BUG=418236
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review URL: https://codereview.chromium.org/1830043002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#382999}

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