Generated writing-mode in ComputedStyleBase

Moved writing-mode to be generated in ComputedStyleBase, which uses the
external type platform/text/WritingMode. Also added functionality to
add 'get' to the front of getters that have names that will conflict
with type names once the Blink rename has happened (and all methods
start with a capital letter).

BUG=628043

Review-Url: https://codereview.chromium.org/2609133002
Cr-Commit-Position: refs/heads/master@{#441271}
4 files changed