Add new subproject to .gitignore.

third_party/mesa/src was added to DEPS in r204765; add to .gitignore.

BUG=
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/15821015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@205749 0039d316-1c4b-4281-b951-d872f2087c98

.gitignore

@@ -225,6 +225,7 @@ v8.log
 /third_party/llvm-allocated-type
 /third_party/llvm-build
 /third_party/lss
+/third_party/mesa/src
 /third_party/mingw-w64
 /third_party/mkl
 /third_party/mt19937ar