[CSS Grid Layout] the "grid-template-areas" is not identified as computable property.

The "grid-template-areas" property is a computable area, defined in the
CSSComputedStyleDeclaration class with an specific computed value. However,
it's not shown by the Web Inspector in the corresponding Computed tab.

It seems that the root cause of this issue is that the property is not
present in the CSSComputedStyleDeclaration::staticComputableProperties.


BUG=79180

Review URL: https://codereview.chromium.org/200263003

git-svn-id: svn://svn.chromium.org/blink/trunk@170318 bbb929c8-8fbe-4397-9dbb-9b2b20218538