Add missing virtual keyword for methods with OVERRIDE

Several methods are marked as OVERRIDE but they are missing the virtual
keyword. This CL explicitly marks those methods as virtual for clarity.

R=eseidel, esprehn
BUG=333316

Review URL: https://codereview.chromium.org/135833002

git-svn-id: svn://svn.chromium.org/blink/trunk@164943 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Regression(r164927): gcc 4.8 build with C++11 support is broken

Fix C++11 build with gcc 4.8 by moving the OVERRIDE before the
WTF_ATTRIBUTE_PRINTF(). Although clang did not seem to have trouble with it,
gcc refuses to compile unless OVERRIDE is right after the method method and
before the WTF_ATTRIBUTE_PRINTF().

R=eseidel, abarth
BUG=333316

Review URL: https://codereview.chromium.org/135883002

git-svn-id: svn://svn.chromium.org/blink/trunk@164944 bbb929c8-8fbe-4397-9dbb-9b2b20218538