This will allow easier testing of each property as we add support in
CSS Typed OM, as we will not have to hand-write exhaustive tests for each
property, and we will easily be able to add a new test that applies to
every element easily if we need to later.

The standard set of tests is:

*single-valued properties*
- Setting with a sequence of valid values throws
- Calling append with a valid value throws

*list-valued properties*
- Setting with a sequence of valid values works
- Setting with a sequence containing valid and invalid values throws
- Append succeeds with a valid value
- Appending a sequence of valid values succeeds
- Append throws when given an invalid value
- Append throws when given a sequence containing valid and invalid values
- GetAll when the property is set to a list returns all the properties

*common*
- Can be set to each valid keyword (automatically adds initial, inherit
    and unset)
- Can be set to each given valid instance of StyleValue
- Setting to an invalid type throws (automatically adds null, undefined,
    true, false, 1, 'hello', {}, and a fake keyword)
- Get returns the right KeywordValue when the property is set to each
    valid keyword
- Get returns the right StyleValue when the property is set to .cssText
    of each given valid StyleValue
- GetAll works when the property is set to a valid StyleValue instance
- Delete removes the style from the element
- GetProperties includes the name of the property when it is set
    to something

I've added example tests for "bottom" and "animation-direction", as
semi-random, convenient single-valued and list-valued properties.

Note that since getting CSSKeyword from a StylePropertyMap is not yet fully
supported, the new tests fail.

BUG=545318

Review-Url: https://codereview.chromium.org/2529043002
Cr-Commit-Position: refs/heads/master@{#435569}