[Impeller] add doc on iOS flamegraph capture. (#49469) I find flamegraphs pretty useful, but Xcode doesn't have one built in that I know of.

diff --git a/impeller/docs/assets/ios_profiling/deep_copy.png b/impeller/docs/assets/ios_profiling/deep_copy.png
new file mode 100644
index 0000000..031eaaa
--- /dev/null
+++ b/impeller/docs/assets/ios_profiling/deep_copy.png
Binary files differ

diff --git a/impeller/docs/assets/ios_profiling/flamegraph.png b/impeller/docs/assets/ios_profiling/flamegraph.png
new file mode 100644
index 0000000..83cbd65
--- /dev/null
+++ b/impeller/docs/assets/ios_profiling/flamegraph.png
Binary files differ

diff --git a/impeller/docs/assets/ios_profiling/ios_profiler_select_thread.png b/impeller/docs/assets/ios_profiling/ios_profiler_select_thread.png
new file mode 100644
index 0000000..87b6bd0
--- /dev/null
+++ b/impeller/docs/assets/ios_profiling/ios_profiler_select_thread.png
Binary files differ

diff --git a/impeller/docs/assets/ios_profiling/ios_time_profiler_example.png b/impeller/docs/assets/ios_profiling/ios_time_profiler_example.png
new file mode 100644
index 0000000..ac2db89
--- /dev/null
+++ b/impeller/docs/assets/ios_profiling/ios_time_profiler_example.png
Binary files differ

diff --git a/impeller/docs/ios_cpu_profile.md b/impeller/docs/ios_cpu_profile.md
new file mode 100644
index 0000000..96a30de
--- /dev/null
+++ b/impeller/docs/ios_cpu_profile.md

@@ -0,0 +1,25 @@
+# iOS CPU Profiling
+
+XCode has a number of built in profiling tools that can easily be used with local or prebuilt engines. This document is focused on creating flame graph charts, which is not a part of XCode but provides a useful way to look at aggregate performance. Compared to the Android Studio flame graph this one is a little wonky, but still useful.
+
+
+1. Clone the "FlameGraph" swift package from `https://github.com/lennet/FlameGraph`
+2. Start a "Time Profiler" based profile session.
+
+![Alt text](assets/ios_profiling/ios_time_profiler_example.png)
+
+  Click the red dot to begin and record as much profile data as you want, then press stop to conclude the trace.
+
+3. Select the thread to investigate, in this case you want io.flutter.raster. IMPORTANT: also select the trace root.
+
+![Alt text](assets/ios_profiling/ios_profiler_select_thread.png)
+
+4. Copy the trace with the keyboard shortcut (⇧⌘C) or the menu ("Edit" -> "Deep Copy").
+
+![Alt text](assets/ios_profiling/deep_copy.png)
+
+5. On the command line, run `swift run FlameGraph --html output.html`
+
+A new browser tab will open with the flame graph. It may require some zooming to be useful.
+
+![Alt text](assets/ios_profiling/flamegraph.png)
\ No newline at end of file