Generate nonproperty 'variableReference' in ComputedStyleBase.

Move the nonproperty field 'variableReference' to be generated in
ComputedStyleBase. Also moved getter / setter methods to be generated in
ComputedStyleBase.

Note that this renames m_variableReference to something much longer.
This shouldn't affect anything since the member is private and only used
within the generated class.

BUG=628043

Review-Url: https://codereview.chromium.org/2676513002
Cr-Commit-Position: refs/heads/master@{#454271}
3 files changed