Remove dawn_build_litert_lm gn arg

Remove the dawn_build_litert_lm gn arg since it isn't built by default
anyways. Also add a helpful check to the build script to detect if
LiteRT-LM source is missing to inform user that the gclient flag is
needed.

Bug: 521332988
Change-Id: I837daf46b4ef780ade6c1fa6e10547ec08d7ddbb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/322055
Commit-Queue: Natalie Chouinard <chouinard@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 3d5b891..ced76d6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -103,11 +103,9 @@
     deps = [ "tools/android" ]
   }
 
-  if (dawn_build_litert_lm) {
-    group("litert_lm") {
-      testonly = true
-      deps = [ "third_party/litert-lm:litert_lm_advanced_main" ]
-    }
+  group("litert_lm") {
+    testonly = true
+    deps = [ "third_party/litert-lm:litert_lm_advanced_main" ]
   }
 
   if (dawn_build_emdawnwebgpu) {

scripts/dawn_features.gni

diff --git a/scripts/dawn_features.gni b/scripts/dawn_features.gni
index 4390d48..05e4290 100644
--- a/scripts/dawn_features.gni
+++ b/scripts/dawn_features.gni
@@ -33,9 +33,6 @@
   # Sets defaults for other features so that we build as much as possible on the
   # current host OS.
   dawn_maximal = false
-
-  # Build the LiteRT-LM benchmark binary using Bazel.
-  dawn_build_litert_lm = false
 }
 
 declare_args() {

third_party/litert-lm/build_litert_lm.py

diff --git a/third_party/litert-lm/build_litert_lm.py b/third_party/litert-lm/build_litert_lm.py
index f963513..4cc3f39 100644
--- a/third_party/litert-lm/build_litert_lm.py
+++ b/third_party/litert-lm/build_litert_lm.py
@@ -67,6 +67,16 @@
     project_root = script_dir.parent.parent
     litert_lm_dir = script_dir / 'src'
 
+    if not litert_lm_dir.exists():
+        print("Error: LiteRT-LM source directory not found.", file=sys.stderr)
+        print(
+            "Please add the following to your .gclient file under 'custom_vars':",
+            file=sys.stderr)
+        print('    "checkout_litert_lm": True,', file=sys.stderr)
+        print("And then run `gclient sync` to download the dependencies.",
+              file=sys.stderr)
+        sys.exit(1)
+
     bazelisk_path = project_root / 'tools' / 'bazelisk' / 'bazelisk'
     prebuilt_cipd_dir = script_dir / 'data' / 'prebuilt'
     prebuilt_src_dir = litert_lm_dir / 'prebuilt'