Add support for base histograms in histograms.xml.
A base histogram is a histogram that is intended to be suffixed,
but that shouldn't itself show up in the histogram UI. A base
histogram is conceptually similar to an abstract base class.
The histograms.xml syntax is:
<histogram base="true" name="PageLoad.AbortTiming.Background" units="ms">
A similar syntax is added for marking suffixes as 'base':
<suffix base="true" name="AfterPaint.BeforeInteraction"/>
PageLoad.AbortTiming base histograms do not record samples, so we
want to avoid showing them in the UI. To do so, we mark them
as base histograms using the new 'base' histogram attribute.
For now, 'base' histograms are simply marked as 'obsolete' with a
default obsolete reason 'Base histogram. Use suffixes of this
histogram instead.'. In the future, we can consider plumbing the
'base' information through to the UI using a mechanism other than
the 'obsolete' reason, if a reason to do so emerges.
BUG=656119
Review-Url: https://codereview.chromium.org/2426473006
Cr-Commit-Position: refs/heads/master@{#426087}
3 files changed
