bug 1335343 - Add initial record_in_processes support r=Dexter

The probes needing to record in gpu were determined by listing the probes that
submitted data for those measures on Nightly on April 18.

MozReview-Commit-ID: 85nQA8rCH1p

toolkit/components/telemetry/docs/collection/histograms.rst

@@ -106,6 +106,7 @@ The following is a sample histogram declaration from ``Histograms.json`` for a h
 .. code-block:: json
 
     "MEMORY_RESIDENT": {
+      "record_in_processes": ["main", "content"],
       "alert_emails": ["team@mozilla.xyz"],
       "expires_in_version": "never",
       "kind": "exponential",
@@ -120,6 +121,16 @@ Histograms which track timings in milliseconds or microseconds should suffix the
 
 The possible fields in a histogram declaration are listed below.
 
+``record_in_processes``
+-----------------------
+Required. This field is a list of processes this histogram can be recorded in. Currently-supported values are:
+
+- ``main``
+- ``content``
+- ``gpu``
+- ``all_child`` (record in all child processes)
+- ``all`` (record in all processes)
+
 ``alert_emails``
 ----------------
 Required. This field is a list of e-mail addresses that should be notified when the distribution of the histogram changes significantly from one build-id to the other. This can be useful to detect regressions. Note that all alerts will be sent automatically to mozilla.dev.telemetry-alerts.