Add third-party library 'webtreemap' for use in tools.

BUG=

Review URL: https://codereview.chromium.org/131463006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245353 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245353