Split initial methods of margin on ComputedStyle.

Currently the margin properties share the same method for the initial
value: initialMargin(). This makes it difficult to generate
because the generator will generate four copies of initialMargin(),
causing a name conflict.

This patch splits the initialMargin() method into four separate methods,
one for each property.

BUG=628043

Review-Url: https://codereview.chromium.org/2825663002
Cr-Commit-Position: refs/heads/master@{#465490}
2 files changed