gitignore: Add tools/histograms/.
gitignore: Add tools/histograms/.

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

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

git-svn-id: http://src.chromium.org/svn/trunk/src@56885 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 20bcf45ae297ffbf02cda2db768bda46cbc9dc26
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser