Change the user action file format from .txt to .xml.

In this way, more information can be added (currently added 'description' and 'owner' for each action) 

A few functions are moved from tools/metrics/histograms to tools/metrics/common to be shared by tools/metrics and tools/histograms.

BUG=340735
NOTRY=true

Review URL: https://codereview.chromium.org/149503005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255357 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255357