Update {virtual,override} to follow C++11 style in src/pipe.

Update {virtual,override} to follow C++11 style in src/pipe.

The Google style guide states that only one of {virtual,override,final}
should be used for each declaration, since override implies virtual and
final implies both virtual and override.

The entries were found using the following command line:

$ find src/pipe -iname "*.h" -or -iname "*.cpp" | xargs pcregrep -M
"[^\n/]+virtual\ [^;{]+\ [a-zA-Z0-9_]+\([^;{]+\ override[ \n]*[;{]"

The regex was a courtesy of nick@chromium.org

BUG=None
R=mtklein@google.com

Review URL: https://codereview.chromium.org/1104863003