[doc] add more details how to debug with lldb

Documents how to use lldb to debug Chromium processes

Change-Id: I39beec3addc5c9360a0120e4d8d1f4e9e46a390a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153074
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759972}

docs/lldbinit.md

@@ -11,3 +11,23 @@ To use, add the following to your `~/.lldbinit`
 script sys.path[:0] = ['<.../path/to/chromium/src/tools/lldb>']
 script import lldbinit
 ```
+
+## How to attach to a process with lldb and start debugging
+
+- Follow the instructions above to create your `~/.lldbinit` file, don't forget to put the correct path to Chromium source in there.
+- Inside of your Chromium checkout, run `lldb out/Default/chrome` (or `out/Debug/chrome`)
+    - On Mac, most likely, `lldb out/Default/Chromium.app/Contents/MacOS/Chromium`
+- Keep lldb running and start Chromium separately with `--no-sandbox` flag:
+    - On Linux, `out/Default/chrome --no-sandbox`
+    - On Mac, `out/Default/Chromium.app/Contents/MacOS/Chromium --no-sandbox`
+    - Note: if you start the process from lldb using `process launch -- --no-sandbox`, you will attach to the main browser process and will not be able to debug tab processes.
+- In Chromium, go to Customize and Control Chromium (three dots) -> More Tools -> Task Manager
+- Depending on what tab or process you want to debug, note the process ID.
+- In the lldb shell:
+    - Execute `process attach -p PID`. PID is the process ID of the tab (process) you want to debug.
+        - Note: it might take a while. Once lldb attaches to the process, you will see a message `Process PID stopped` and some stack traces.
+    - Now you can set breakpoints, for example, `breakpoint set -f inspector_overlay_agent.cc -l 627`.
+    - Execute `cont` to continue the execution of the process.
+    - Perform the actions which would trigger the breakpoint. lldb will stop the execution for you to inspect.
+    - You can pause the execution at any time by pressing Ctrl + C.
+    - Type `help` to learn more about different lldb commands.