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Currently, UIDevToolsTest tests the whole system together
in a single test file. This file uses WindowElement in most
tests, which makes it difficult to port for the forthcoming
Mac implementation, which does not have WindowElement due
to not being an Aura platform.

This  is the first of a series of changes that will
break most of UIDevToolsTest up into more granular tests,
which will make it easier to isolate WindowElement use
to Aura.

This change takes the functionality that was previously
being tested in the style tests in UIDevToolsTest, and
breaks it into:
- css_agent_unittest.cc, which tests interaction between
elements and CSSAgent
- Separate files for individual element subclasses, to
ensure the mapping between the elements and their underlying
objects.

Additionally, it fixes a few bugs smoked out by the new
tests in CSSAgent and WidgetElement.

Bug: 769352
Change-Id: Ie24f2166a2dbe7fef31db2800c8bf88a1f470057
Reviewed-on: https://chromium-review.googlesource.com/1231033
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592401}