[cronet] Exclude unused classes to slim binary

Cronet uses only a few classes in base_java. The CL mainly excludes
classes that contain library_loader, multidex and process_launcher.
The size of cronet_impl_native_java.jar is reduced by 165 KiB.

Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester
Change-Id: I10d1546b38538052d2e2e286c95b71d20e7dbeb9
Reviewed-on: https://chromium-review.googlesource.com/955802Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Paul Jensen <pauljensen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542392}

components/cronet/android/BUILD.gn

@@ -30,6 +30,12 @@ generate_jni_registration("cronet_jni_registration") {
   target = ":cronet_sample_apk"
   output = "$root_gen_dir/components/cronet/android/${target_name}.h"
   exception_files = jni_exception_files
+
+  exception_files += [
+    "//base/android/java/src/org/chromium/base/library_loader/LibraryLoader.java",
+    "//base/android/java/src/org/chromium/base/process_launcher/ChildProcessService.java",
+    "//base/android/java/src/org/chromium/base/SysUtils.java",
+  ]
 }
 
 java_cpp_enum("effective_connection_type_java") {
@@ -924,9 +930,18 @@ action("repackage_extracted_native_jars") {
     _output_jar,
   ]
 
+  jar_excluded_patterns = [
+    "*/library_loader/*.class",
+    "*/multidex/*.class",
+    "*/process_launcher/*.class",
+    "*/SysUtils.class",
+    "*/CachedMetrics.class",
+  ]
+
   args = [
     "--classes-dir",
     rebase_path(_extract_cronet_native_jars_dir, root_build_dir),
+    "--excluded-classes=$jar_excluded_patterns",
     "--jar-path",
     rebase_path(_output_jar, root_build_dir),
   ]