Bug 1550692 Process subsection headers in about:telemetry should be clear that they're about processes r=chutten,flod

Differential Revision: https://phabricator.services.mozilla.com/D31718

--HG--
extra : moz-landing-system : lando

toolkit/content/aboutTelemetry.js

@@ -1628,7 +1628,7 @@ class Section {
     if (data && Object.keys(data).length > 0) {
       let s = GenericSubsection.renderSubsectionHeader(process, true, section);
       let heading = document.createElement("h2");
-      heading.textContent = process;
+      document.l10n.setAttributes(heading, "about-telemetry-process", {process});
       s.appendChild(heading);
 
       this.renderData(data, s);

toolkit/locales/en-US/toolkit/about/aboutTelemetry.ftl

@@ -142,3 +142,6 @@ about-telemetry-origin-count = count
 # Variables:
 #   $telemetryServerOwner (String): the value of the toolkit.telemetry.server_owner preference. Typically "Mozilla"
 about-telemetry-origins-explanation = <a data-l10n-name="origin-doc-link">Firefox Origin Telemetry</a> encodes data before it is sent so that { $telemetryServerOwner } can count things, but not know whether or not any given { -brand-product-name } contributed to that count. (<a data-l10n-name="prio-blog-link">learn more</a>)
+# Variables:
+#  $process (String): type of process in subsection headers ( e.g. "content", "parent" )
+about-telemetry-process = { $process } process