gitignore: Add tools/histograms/.

BUG=none
TEST="git status"
TBR=evan@chromium.org

Review URL: http://codereview.chromium.org/3166026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@56885 0039d316-1c4b-4281-b951-d872f2087c98

.gitignore

@@ -96,6 +96,7 @@
 /tools/distcc
 /tools/grit/grit/test/data
 /tools/gyp
+/tools/histograms/
 /tools/page_cycler/acid3
 /tools/tryserver
 /v8