This change adds HistogramFactoryGetCountsLinear and
RTC_HISTOGRAM_COUNTS_LINEAR. Note that the default implementation of
HistogramFactoryGetCounts in metrics_default.cc also provides a
linearly spaced histogram, while the Chrome UMA implementation
provides exponentially spaced buckets.
BUG=none
Review-Url: https://codereview.webrtc.org/2548463002
Cr-Commit-Position: refs/heads/master@{#15367}