base: Remove a copy of and copy ctor from SequenceAndSortKey.

This patch removes a copy of SequenceAndSortKey. As a result, the copy
ctor is no longer needed (there won't be an implicit one generated).

R=danakj, gab

Review URL: https://codereview.chromium.org/1901223003

Cr-Commit-Position: refs/heads/master@{#388340}
3 files changed