More Compiler.h cleanups.

* We support neither gcce nor icc.
* Our gcc is always newer than 3.0.3.
* Our clang always has __has_extension
* UNUSED_LABEL isn't used anywhere

BUG=none

Review URL: https://chromiumcodereview.appspot.com/14734009

git-svn-id: svn://svn.chromium.org/blink/trunk@150429 bbb929c8-8fbe-4397-9dbb-9b2b20218538
4 files changed