Bug 1536175 - Report whether Headless or not in Telemetry Environment r=janerik

If Firefox is in Headless Mode we expect it to behave rather differently. For
example, I imagine gpu perf metrics will be less than representative. We should
annotate Telemetry pings with whether or not we're operating in headless mode.

Differential Revision: https://phabricator.services.mozilla.com/D26970

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

toolkit/components/telemetry/app/TelemetryEnvironment.jsm

@@ -1763,6 +1763,7 @@ EnvironmentCache.prototype = {
       D2DEnabled: getGfxField("D2DEnabled", null),
       DWriteEnabled: getGfxField("DWriteEnabled", null),
       ContentBackend: getGfxField("ContentBackend", null),
+      Headless: getGfxField("isHeadless", null),
       // The following line is disabled due to main thread jank and will be enabled
       // again as part of bug 1154500.
       // DWriteVersion: getGfxField("DWriteVersion", null),

toolkit/components/telemetry/docs/data/environment.rst

@@ -154,6 +154,7 @@ Structure:
         gfx: {
             D2DEnabled: <bool>, // null on failure
             DWriteEnabled: <bool>, // null on failure
+            Headless: <bool>, // null on failure
             //DWriteVersion: <string>, // temporarily removed, pending bug 1154500
             adapters: [
               {

toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js

@@ -611,6 +611,7 @@ function checkSystemSection(data) {
   let gfxData = data.system.gfx;
   Assert.ok("D2DEnabled" in gfxData);
   Assert.ok("DWriteEnabled" in gfxData);
+  Assert.ok("Headless" in gfxData);
   // DWriteVersion is disabled due to main thread jank and will be enabled
   // again as part of bug 1154500.
   // Assert.ok("DWriteVersion" in gfxData);