Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+Instrumenting JavaScript
+========================
+
+There are multiple ways to use the profiler with JavaScript. There is the "JavaScript"
+profiler feature (via about:profiling), which enables stack walking for JavaScript code.
+This is most likely turned on already for every profiler preset.
+
+In addition, markers can be created to specifically marker an instant in time, or a
+duration. This can be helpful to make sense of a particular piece of the front-end,
+or record events that normally wouldn't show up in samples.
+
+.. note::
+    This guide explains JavaScript markers in depth. To learn more about how to add a
+    marker in C++ or Rust, please take a look at their documentation
+    in :doc:`markers-guide` or :doc:`instrumenting-rust` respectively.
+
+Markers in Browser Chrome
+*************************
+
+If you have access to ChromeUtils, then adding a marker is relatively easily.
+
+.. code-block:: javascript
+
+  // Add an instant marker, representing a single point in time
+  ChromeUtils.addProfilerMarker("MarkerName");
+
+  // Add a duration marker, representing a span of time.
+  const startTime = Cu.now();
+  doWork();
+  ChromeUtils.addProfilerMarker("MarkerName", startTime);
+
+  // Add a duration marker, representing a span of time, with some additional tex
+  const startTime = Cu.now();
+  doWork();
+  ChromeUtils.addProfilerMarker("MarkerName", startTime, "Details about this event");
+
+  // Add an instant marker, with some additional tex
+  const startTime = Cu.now();
+  doWork();
+  ChromeUtils.addProfilerMarker("MarkerName", undefined, "Details about this event");
+
+Markers in Content Code
+***********************
+
+If instrumenting content code, then the `UserTiming`_ API is the best bet.
+:code:`performance.mark` will create an instant marker, and a :code:`performance.measure`
+will create a duration marker. These markers will show up under UserTiming in
+the profiler UI.
+
+.. code-block:: javascript
+
+  // Create an instant marker.
+  performance.mark("InstantMarkerName");
+
+  doWork();
+
+  // Measuring with the performance API will also create duration markers.
+  performance.measure("DurationMarkerName", "InstantMarkerName");
+
+.. _UserTiming: https://developer.mozilla.org/en-US/docs/Web/API/User_Timing_API