Update SitePerProcessBrowserTest Page Scale Factor Reading

SitePerProcessBrowserTest had a helper method to lookup the page scale factor.
This was then being used to direct input events for testing.

However it was using the legacy path of cached CompositorFrameMetadata which
does not work in Viz. Leading to some tests incorrectly targetting the wrong
views.

This updates them to use the newer RenderFrameMetadata path which works both in
classic and Viz modes.

TEST=SitePerProcessBrowserTest.ViewBoundsInNestedFrameTest,
SitePerProcessBrowserTest.ScrollBubblingFromNestedOOPIFTest,
SitePerProcessBrowserTest.ScrollLocalSubframeInOOPIF

Bug: 848825
Change-Id: Ie8ce1f6144450c31d2673c55099e68e0979ea9bb
Reviewed-on: https://chromium-review.googlesource.com/1141987
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576650}
3 files changed