Android: Add reminders for android_deps

Add per-target reminders that these targets are generated and
customizations should go into BuildConfigGenerator.groovy instead.

Bug: 913062
Change-Id: I1e6a927378de741efd79e7f248ce714cb1f55dd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525310
Commit-Queue: Peter Wen <wnwen@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Auto-Submit: Peter Wen <wnwen@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#641634}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e8a62b66f380b2bc50b673605e999f4701a40fb7
diff --git a/roll/android_deps/buildSrc/src/main/groovy/BuildConfigGenerator.groovy b/roll/android_deps/buildSrc/src/main/groovy/BuildConfigGenerator.groovy
index fc14fcc..8fbb3ab 100644
--- a/roll/android_deps/buildSrc/src/main/groovy/BuildConfigGenerator.groovy
+++ b/roll/android_deps/buildSrc/src/main/groovy/BuildConfigGenerator.groovy
@@ -24,6 +24,7 @@
     private static final BUILD_GN_GEN_PATTERN = Pattern.compile(
             "${BUILD_GN_TOKEN_START}(.*)${BUILD_GN_TOKEN_END}",
             Pattern.DOTALL)
+    private static final BUILD_GN_GEN_REMINDER = "# This is generated, do not edit. Update BuildConfigGenerator.groovy instead.\n"
     private static final DEPS_TOKEN_START = "# === ANDROID_DEPS Generated Code Start ==="
     private static final DEPS_TOKEN_END = "# === ANDROID_DEPS Generated Code End ==="
     private static final DEPS_GEN_PATTERN = Pattern.compile(
@@ -136,6 +137,7 @@
             }
 
             def libPath = "${DOWNLOAD_DIRECTORY_NAME}/${dependency.id}"
+            sb.append(BUILD_GN_GEN_REMINDER)
             if (dependency.extension == 'jar') {
                 sb.append("""\
                 java_prebuilt("${dependency.id}_java") {