Allow text expectation for reftests

Paint invalidation tests need to check
- what are invalidated (as text) to ensure no over invalidations;
- the result of invalidation and paint to ensure
  - no under-invalidation;
  - we paint the invalidated parts correctly.

We can achieve this with both text and pixel expectations. As reftest
is a better way for pixel expectations, -expected.txt and
-expected.html seem a reasonable combination of expectations.


Cr-Commit-Position: refs/heads/master@{#394667}
8 files changed