Add documentation for generating/using PGO profiles

Change-Id: I733621250e2a873a6e8e68bb332355160e7b082d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490544Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: Arthur Eubanks <aeubanks@google.com>
Cr-Commit-Position: refs/heads/master@{#820877}

docs/pgo.md

+# Generating PGO Profiles
+
+Normally devs don't need to worry about this and can use the default profile
+for official builds.  The default profile can be fetched by adding
+`"checkout_pgo_profiles": True` to `custom_vars` in the gclient config and
+running `gclient runhooks`.
+
+To produce an executable built with a custom PGO profile:
+
+* Produce the instrumented executable using the following gn args:
+
+  ```
+  chrome_pgo_phase = 1
+  enable_resource_allowlist_generation = false
+  is_official_build = true
+  symbol_level = 0
+  use_goma = true
+  ```
+
+* Run representative benchmarks to produce profiles
+
+  * `vpython tools/perf/run_benchmark system_health.common_desktop --assert-gpu-compositing --run-abridged-story-set --browser=exact --browser-executable=out/path/to/chrome`
+  * `vpython tools/perf/run_benchmark speedometer2 --assert-gpu-compositing --browser=exact --browser-executable=out/path/to/chrome`
+  * `vpython tools/perf/run_benchmark webrtc --assert-gpu-compositing --browser=exact --browser-executable=out/path/to/chrome`
+  * This will produce `*.profraw` files in the current working directory
+
+* Merge the profiling data
+
+  * Get the `llvm-profdata` tool by adding `"checkout_clang_coverage_tools": True,` to `custom_vars` in the gclient config and running `gclient runhooks`.
+  * Run `third_party/llvm-build/Release+Asserts/bin/llvm-profdata merge *.profraw -o chrome.profdata`
+
+* Produce the final PGO'd executable with the following gn args:
+
+  ```
+  enable_resource_allowlist_generation = false
+  is_official_build = true
+  symbol_level = 0
+  use_goma = true
+  pgo_data_path = {path-to-the-profile}
+  ```