Thank you, yes, that's whom I meant. 

My current thinking is to place two different metrics. One, UseCounter based, to record per page whether any shaping resulted in any .notdefs to see how often we have a .notdef impression as a ratio of all page views. Second, a metric in our shaping code that records a histogram of graphemes or only the initial characters for which no glyphs were found. What do you think, Sascha & Emil?

Sounds great!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a3976985653410d2d753c268f8085873b6ed4a56

commit a3976985653410d2d753c268f8085873b6ed4a56
Author: drott <drott@chromium.org>
Date: Thu May 11 11:23:59 2017

Report new UseCounter metric when observing NotDef glyphs

When shaping fails to find at least one glyph, report this as a new
UseCounter metric "FontShapingNotDefGlyphObserved" for the current
document, which we can access via FontSelector. Measuring it as a ratio
against all UseCounter events should help us evaluate how frequently
.notdef glyphs occur in general. A separate histogram to be added later
will help find out for which characters or graphemes specifically we
fail to find glyphs for.

BUG=699578

Review-Url: https://codereview.chromium.org/2751163002
Cr-Commit-Position: refs/heads/master@{#470917}

[modify] https://crrev.com/a3976985653410d2d753c268f8085873b6ed4a56/third_party/WebKit/Source/core/css/CSSFontSelector.cpp
[modify] https://crrev.com/a3976985653410d2d753c268f8085873b6ed4a56/third_party/WebKit/Source/core/css/CSSFontSelector.h
[modify] https://crrev.com/a3976985653410d2d753c268f8085873b6ed4a56/third_party/WebKit/Source/core/frame/UseCounter.h
[modify] https://crrev.com/a3976985653410d2d753c268f8085873b6ed4a56/third_party/WebKit/Source/platform/fonts/Font.cpp
[modify] https://crrev.com/a3976985653410d2d753c268f8085873b6ed4a56/third_party/WebKit/Source/platform/fonts/Font.h
[modify] https://crrev.com/a3976985653410d2d753c268f8085873b6ed4a56/third_party/WebKit/Source/platform/fonts/FontSelector.h
[modify] https://crrev.com/a3976985653410d2d753c268f8085873b6ed4a56/third_party/WebKit/Source/platform/fonts/shaping/HarfBuzzShaper.cpp
[modify] https://crrev.com/a3976985653410d2d753c268f8085873b6ed4a56/third_party/WebKit/Source/platform/fonts/shaping/HarfBuzzShaper.h
[modify] https://crrev.com/a3976985653410d2d753c268f8085873b6ed4a56/third_party/WebKit/Source/platform/testing/FontTestHelpers.cpp
[modify] https://crrev.com/a3976985653410d2d753c268f8085873b6ed4a56/tools/metrics/histograms/enums.xml