Changed EEmptyCells to an enum class and renamed its members to keywords

Changed EEmptyCells to an enum class and gave it an unsigned underlying
type. Also renamed its members to match its keywords from
CSSValueKeywords.in. This is pre-work to move EEmptyCells to be
generated in ComputedStyleBase.

BUG=628043
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2367273003
Cr-Commit-Position: refs/heads/master@{#422106}
5 files changed