Make ComputedStyleBase abstract.

Currently ComputedStyleBase can be instantiated, and a ComputedStyle can
be deleted via a pointer to ComputedStyleBase. Both of these could cause
issues since ComputedStyleBase should be an abstract class.

This patch moves the ComputedStyleBase constructor and destructor to be
protected so that only ComputedStyle can call them.

BUG=628043

Review-Url: https://codereview.chromium.org/2739553003
Cr-Commit-Position: refs/heads/master@{#457579}
1 file changed