Remove 'const' prefix from functions that return BorderValue by value.

There are several functions that create a BorderValue and return it by
value. However, the return type is 'const BorderValue' not 'BorderValue'
which makes it confusing because the 'const' is extraneous. This patch
removes the 'const' prefix.

BUG=710938

Review-Url: https://codereview.chromium.org/2878823002
Cr-Commit-Position: refs/heads/master@{#471242}
2 files changed