Tests comparing original-vs-saved for multi-frame pages.

Extra tests navigate to the saved file and verify if it "looks" the same
as the original page (i.e. if it has the same number of frames, if it
has the expected substrings from all subframes, if it has no "err"
substrings).

The extra verification done by the new test catches some regressions
that older tests would not catch (i.e. without the new tests verifying
about:blank regressions is tricky, as substrings for subframes can
appear in the saved file but end up not being rendered because of
incorrect cross-referencing).  Additionally the extra verification adds
coverage for newly discovered bugs (i.e. for handling of <object>
elements).

More details about the changes:

- The new resources got created manually from scratch:
  pdf.pdf and svg.svg using Inkscape,
  png.png using Gimp.

- iframes.htm got renamed to frames-xsite.htm
  (to be named consistently with the other 2 test files in this CL).

- local-about-blank-subframes.htm got renamed to frames-about-blank.htm
  and moved from content/test/data/download into
  chrome/test/data/save_page/

  The test using this file in mhtml_generation_browsertest.cc got
  removed - the coverage offered by one of the new tests
  (SavePageMultiFrameBrowserTest.AboutBlank_MHTML) is a superset of the
  coverage from the old test.

- Content verification markers got tweaked so the tests
  are easier to read (i.e. it is more apparent which htm test file
  the given guid comes from).

- The SavePageMultiFrameBrowserTest to some extent overlap coverage of
  existing SavePageSitePerProcessBrowserTest.  This is ok, as the old and
  new tests still verify some things unique to a specific test.

- In theory we could reuse SavePageSitePerProcessBrowserTest class.
  I've put the new tests into a separate SavePageMultiFrameBrowserTest
  class, because all new tests perform the same steps and verification
  and I plan to parametrize SavePageMultiFrameBrowserTest in the future
  by having it inherit from ::testing::WithParamInterface<SavePageType>.

BUG=553478, 539936, 538766, 526786

Review URL: https://codereview.chromium.org/1412733017

Cr-Commit-Position: refs/heads/master@{#359351}
12 files changed