tools/valgrind/waterfall.sh: Only fetch the 3 most recent builds

This speeds up the "fetch" command by a lot and also it makes sense to ignore
reports that happened long ago.
Also update the filter ("Failure" vs "failed") as the HTML has changed a bit.

TBR=glider
NOTRY=true
Review URL: https://codereview.chromium.org/12096058

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@179589 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed