[css-typed-om] Use create* helper functions in tests.

Currently, to test style property maps, we have to set up some divs
in the body with the test styles. Each test case would then query the
style maps of these divs.

To simplify things, we add two helper functions: createInlineStyleMap
and createComputedStyleMap. Each take a string containing css and will
return an inline/computed StylePropertyMap with that css. This adds
greater isolation between test cases, simplifies tests, and will
help write succinct tests for the declared style property map, which
has a lot of boilerplate.

Bug: 774887
Change-Id: I0ffc571e37601fea02f9d27ef458e3c569ba58cf
Reviewed-on: https://chromium-review.googlesource.com/835887
Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526429}