Changed EDisplay to an enum class and renamed its members to be keywords

Changed EDisplay to an enum class and gave it an unsigned underlying
type. Also renamed its members to match its keywords (e.g. inline,
block, listItem)  This is pre-work to move EDisplay to be generated in
ComputedStyleBase.

BUG=628043

Review-Url: https://codereview.chromium.org/2370673002
Cr-Commit-Position: refs/heads/master@{#422023}
52 files changed