Use default copy constructor for ComputedStyleBase.

The current copy constructor ComputedStyleBase doesn't do anything
special. This patch removes it so we can just let C++ generate the
default copy constructor instead. When we have opportunities for faster
copy in the future (e.g. using memcpy), we will generate the copy
constructor explicitly.

BUG=628043

Review-Url: https://codereview.chromium.org/2744693002
Cr-Commit-Position: refs/heads/master@{#457637}
1 file changed