Update docs to deprecate goma and show warning in gn gen

Test: Tested that `gn gen` prints a warning if `use_goma=true` in args.gn

Bug: b/210975050
Change-Id: Iacfb4edbaf9b06fd693bd681225a7e86ad3e93c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4850412
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Ben Segall <bentekkie@google.com>
Cr-Commit-Position: refs/heads/main@{#1201582}

build/toolchain/goma.gni

@@ -18,3 +18,22 @@ if (use_goma && goma_dir == "") {
 
 assert(!is_win || !use_goma || is_clang,
        "cl.exe does not work on goma, use clang")
+
+if (use_goma) {
+  if (host_os == "win") {
+    print("WARNING: use_goma=true will be deprecated by EOY 2023. " +
+          "Please use `use_remoteexec=true` instead. See " +
+          "https://chromium.googlesource.com/chromium/src/+/main/docs/" +
+          "windows_build_instructions.md#use-reclient for more info.")
+  } else if (host_os == "mac") {
+    print("WARNING: use_goma=true will be deprecated by EOY 2023. " +
+          "Please use `use_remoteexec=true` instead. " +
+          "If you are a googler see " +
+          "go/building-chrome-mac#using-reclient-for-build for more info.")
+  } else {
+    print("WARNING: use_goma=true will be deprecated by EOY 2023. " +
+          "Please use `use_remoteexec=true` instead. See " +
+          "https://chromium.googlesource.com/chromium/src/+/main/docs/" +
+          "linux/build_instructions.md#use-reclient for more info.")
+  }
+}

docs/windows_build_instructions.md

@@ -232,6 +232,21 @@ local variable or type information. With `symbol_level = 0` there is no
 source-level debugging but call stacks still have function names. Changing
 `symbol_level` requires recompiling everything.
 
+#### Use Reclient
+
+In addition, Google employees should use reclient, a distributed compilation system.
+Detailed information is available internally but the relevant gn arg is:
+* `use_remoteexec = true`
+
+Google employees can visit go/building-chrome-win#setup-reclient for more information.
+
+When invoking ninja, specify 'chrome' as the target to avoid building all test
+binaries as well.
+
+Still, builds will take many hours on many machines.
+
+#### Use Goma (deprecated)
+
 In addition, Google employees should use goma, a distributed compilation system.
 Detailed information is available internally but the relevant gn arg is:
 * `use_goma = true`
@@ -244,7 +259,7 @@ will automatically pass an appropriate -j value to ninja for goma or not.
 $ autoninja -C out\Default chrome
 ```
 
-When invoking ninja specify 'chrome' as the target to avoid building all test
+When invoking ninja, specify 'chrome' as the target to avoid building all test
 binaries as well.
 
 Still, builds will take many hours on many machines.