Making r8 on by default for all builds

Even if a proguard_jar_path is set, someone must manually set use_r8 = false
in their GN args to use it.

Bug: 908988
Change-Id: I509db915cba46ad694b687eda8a3b0123234847c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1689877Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676164}

build/config/android/config.gni

@@ -214,10 +214,9 @@ if (is_android || is_chromeos) {
     android_sdk_tools_bundle_aapt2_dir =
         "//third_party/android_build_tools/aapt2"
 
-    # Use R8 for Java optimization rather than ProGuard for all targets. R8 is
-    # already used as the default for public targets. This will evenutally be
-    # the default. https://crbug.com/908988
-    use_r8 = false
+    # Use R8 for Java optimization rather than any progaurd_jar_path provided
+    # for all targets. Proguard support may be dropped in the near future.
+    use_r8 = true
 
     # Checks that proguard flags have not changed (!is_java_debug only).
     check_android_configuration = false