Generate keyword part of vertical-align property in ComputedStyleBase.

The vertical-align property can either have a keyword value or a length
value. The two values are stored separately in ComputedStyle. The
keyword part is stored directly in ComputedStyle, but the length part is
stored in StyleBoxData. Hence, generating this property is quite
difficult.

This patch generates just the keyword part of vertical-align. To make it
clear that this is not generating the whole of vertical-align properly,
we generate it as a 'storage_only' nonproperty.

Diff of generated files:
https://gist.github.com/3e52ce1e0e38fd208196ed0621f6c0b3/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2775573002
Cr-Commit-Position: refs/heads/master@{#460283}
3 files changed