Fix separator to use . for SameVersionStartupCounts suffix.
Fix separator to use . for SameVersionStartupCounts suffix.

The code already uses ".", forgot to specify in histograms.xml

BUG=580211
TBR=jwd@chromium.org

Review URL: https://codereview.chromium.org/1643323002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#372439}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk