More cross-platform pixel test support in MockPrinter.

It used to only work on Mac (maybe; no pixels were actually tested
due to no test coverage), and possibly Windows as well, using platform
code. It should now work in Linux, Mac and Windows, but not Android
(because PDF isn't enabled there).

Remove platform-specific code from printing::Image. Instead provide a
constructor that takes raw pixel data. Generate the raw pixel data
from PDF in MockPrinter.

MockPrinter::PrintPage() was confused (now named OnDocumentPrinted()).
It expected to be called once per page, but it is in fact only called
once per print job. Not that it really mattered, since no tests tested
more than one page. :-p

Enable PrintRenderFrameHelperTest.PrintWithIframe on all platforms that
support pixel testing.
Remove PrintRenderFrameHelperTest.PrintLayoutTest, since it's not very
useful. It was only enabled on Mac, and it essentially only tested if
the page size was *roughly* as expected. We can do better than this
now. Add PrintRenderFrameHelperTest.Pixels, which checks all the
pixels in a two-page printed document with mixed page size.

Change-Id: If275acb230b5834c0f843365d0ea9dfa26a70e78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4765574
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1183555}

• components/printing/test/BUILD.gn[diff]
• components/printing/test/DEPS[diff]
• components/printing/test/mock_printer.cc[diff]
• components/printing/test/mock_printer.h[diff]
• components/printing/test/print_render_frame_helper_browsertest.cc[diff]
• printing/BUILD.gn[diff]
• printing/image.cc[diff]
• printing/image.h[diff]
• printing/image_android.cc[Deleted - diff]
• printing/image_linux.cc[Deleted - diff]
• printing/image_mac.cc[Deleted - diff]
• printing/image_win.cc[Deleted - diff]