Refactor Histogram relationship diagnostics.

docs/histogram-set-json-format.md

 <!-- Copyright 2016 The Chromium Authors. All rights reserved.
      Use of this source code is governed by a BSD-style license that can be
      found in the LICENSE file.
 -->
 
 # HistogramSet JSON Format
 
 This document assumes familiarity with the concepts introduced in
 [how-to-write-metrics](/docs/how-to-write-metrics.md).
 
@@ skipping 43 matching lines @@
     "values": ["browse:news:cnn"],
   },
 ]
 ```
 
 ## Histograms
 
 ### Required fields
 
  * `name`: any string
- * `guid`: string UUID, allows Histograms to reference other Histograms via
-   RelatedHistogram Diagnostics
+ * `guid`: string UUID
  * `unit`: underscore-separated string of 1 or 2 parts:
     * The required unit base name must be one of
        * ms
        * tsMs
        * n%
        * sizeInBytes
        * J
        * W
        * unitless
        * count
@@ skipping 42 matching lines @@
 
 DiagnosticMap is a dictionary mapping strings to Diagnostic dictionaries.
 
 ## Diagnostics
 
 The only field that is required for all Diagnostics, `type`, must be one of
  * `Breakdown`
  * `DateRange`
  * `GenericSet`
  * `RelatedEventSet`
- * `RelatedHistogramBreakdown`
- * `RelatedHistogramMap`
+ * `RelatedNameMap`
  * `Scalar`
 
 If a Diagnostic is in the root array of the JSON, then it is shared, so it may be
 referenced by multiple Histograms. Shared Diagnostics must contain a string
 field `guid` containing a UUID.
 
 If a Diagnostic is contained in a Histogram, then it must not have a `guid`
 field.
 
 The other fields of Diagnostic dictionaries depend on `type`.
@@ skipping 20 matching lines @@
  * `values`: array of any JSON data.
 
 ### RelatedEventSet
 
 This allows metrics to explain the magnitude of a sample as a parameter of a
 specific event or set of events in a trace.
 
  * `events`: array of dictionaries containing `stableId`, `title`, `start`,
    `duration` fields of Events
 
-### RelatedHistogramMap
 
-This allows metrics to annotate which Histograms are related to other
-Histograms, and annotate the nature of the relationship with a custom string
-name.
+### RelatedNameMap
 
- * `values`: dictionary mapping from custom string name to the related
-   Histogram's string guid
+This is a Map from short descriptive names to full Histogram names.
 
-### RelatedHistogramBreakdown
-
-This allows metrics to explain the magnitudes of the samples in a Histogram
-collectively as composed of various categories.
-
- * `values`: dictionary mapping from custom string name to the related
-   Histogram's string guid
+ * `names`: a dictionary mapping strings to strings containing Histogram names.
 
 ### Scalar
 
 Metrics should not use Scalar diagnostics since they cannot be safely merged.
 
  * `value`: a dictionary containing a string `unit` and a number `value`