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Currently, PdfMetafileCgTest.RenderPageBasic does all the rendering
and bitmap comparisons directly within the test. PdfMetafileCgTest will
add more tests to its suite that will do the same things. Add helpers to
simplify the addition of tests.

Change-Id: I1973d04d8a93cf1f1e1f19ff55eb6af66316cb7a
Bug: 919596
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125227
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755242}