Add gdbserver instructions to site docs

Change-Id: I13ba477d5b390a0fc03cd4960b6a19bcaa4266d7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/482699
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Commit-Queue: Joe Gregorio <jcgregorio@google.com>

site/docs/dev/tools/android_gdb.md

@@ -0,0 +1,62 @@
+
+---
+title: "Debugging DM on Android"
+linkTitle: "Debugging DM on Android"
+
+---
+
+By default, we don’t do Android builds with full symbols. Assuming you want more than callstacks,
+add the following to your GN args:
+
+~~~
+extra_cflags = [ "-g" ]
+~~~
+
+When you build, you need to have built the `gdbserver` target. Either build everything, or at least
+build both `dm` and `gdbserver`:
+
+<!--?prettify lang=sh?-->
+
+    ninja -C out/android dm gdbserver
+
+At this point, the Android gdb script should work. Try running:
+
+<!--?prettify lang=sh?-->
+
+    platform_tools/android/bin/android_gdb_native -C out/android dm -i /data/local/tmp/resources <args>
+
+You will end up in a command-line gdb session connected to dm on the device. You’re done, but you
+can do better. From here, I’m assuming that you use VS Code. I strongly suspect that this could be
+adapted to other IDEs' GDB integration.
+
+VS Code comes with lldb support, but this workflow needs a GDB extension. Search for 'Native Debug'
+in the extension browser and install it, the thing you want comes from
+https://github.com/WebFreak001/code-debug if you’re unsure.
+
+In your VS Code project’s `launch.json`, add an entry that looks like the following. You'll need to
+replace <NDK_BUNDLE> with the path to your NDK bundle (ie $ANDROID_NDK_ROOT):
+
+~~~
+{
+    "name": "Android GDB",
+    "type": "gdb",
+    "request": "attach",
+    "target": ":5039",
+    "remote": true,
+    "gdbpath": "<NDK bundle>/prebuilt/linux-x86_64/bin/gdb",
+    "executable": "out/android/android_gdb_tmp/dm",
+    "cwd": "${workspaceRoot}",
+    "autorun": [ "break main" ]
+}
+~~~
+
+Rather than running `android_gdb_native`, run `android_gdbserver` in the same directory (and with
+the same arguments). This will do all of the same deployment, and run `gdbserver` on the device,
+but won’t start command line gdb on your host.
+
+Now, just 'Start Debugging' in VS Code (with the new configuration selected, if you have more than
+one). The VSC hosted gdb will connect to gdbserver, and you’ll have a somewhat interactive debugger
+where you can set breakpoints in your source windows, have panes for watches, locals, and the call
+stack, etc. Enjoy:
+
+![VS Code Debugger Screenshot](../android_gdb.png)