metrics: add DocumentScan processing metrics
metrics: add DocumentScan processing metrics

Add a histogram "DocumentScan.ScanResult" and
"DocumentScan.ConverterResult" for tracking success of scanning
operations within the ChromeOS lorgnette daemon.

BUG=chromium:484325
TEST=pretty_print.py, validate_format.py
R=jwd@chromium.org

Review URL: https://codereview.chromium.org/1124483003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#328578}

TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,Fireblend/chromium-crosswalk,Fireblend/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,Chilledheart/chromium,Just-D/chromium-1,axinging/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,chuan9/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,PeterWangIntel/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,Fireblend/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Fireblend/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Fireblend/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,Fireblend/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,Chilledheart/chromium,TheTypoMaster/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Fireblend/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,axinging/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Just-D/chromium-1,TheTypoMaster/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Fireblend/chromium-crosswalk,Just-D/chromium-1,Chilledheart/chromium,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,Chilledheart/chromium,TheTypoMaster/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Fireblend/chromium-crosswalk,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,chuan9/chromium-crosswalk