android: build: Enable RELR packed relocations for libchrome.so

NOTE: This is a reland of [1], but this one does not touch
libchromefortest.so which is used on Android N+ during testing,
and could not be loaded by the system linker when using RELR
relocations.

This optimization cannot be applied to libmonochrome.so, unfortunately,
because only Android P and above support these at the system linker
level.

[1] https://chromium-review.googlesource.com/c/1304487

BUG=895194
R=agrieve@chromium.org,torne@chromium.org,pasko@chromium.org,pcc@chromium.org

Change-Id: If50f607d2e9abbff5ae0c140da342af7b8c78858
Reviewed-on: https://chromium-review.googlesource.com/c/1317630Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Commit-Queue: David Turner <digit@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609142}

build/config/android/BUILD.gn

@@ -197,6 +197,12 @@ config("lld_pack_relocations") {
   ldflags = [ "-Wl,--pack-dyn-relocs=android" ]
 }
 
+# Pack relocations with Android-specific relocations and RELR relocations that
+# are only supported by the crazy linker and Android P+.
+config("lld_pack_relocations_with_relr") {
+  ldflags = [ "-Wl,--pack-dyn-relocs=android+relr,--use-android-relr-tags" ]
+}
+
 # Used for instrumented build to generate the orderfile.
 config("default_orderfile_instrumentation") {
   if (use_order_profiling) {

chrome/android/BUILD.gn

@@ -979,12 +979,16 @@ jinja_template_resources("chrome_test_apk_template_resources") {
 #      ELF relocations to reduce native library size.
 #    use_gnu_hash_style: If true, the final library will include only GNU hash
 #      tables, rather than both SysV and GNU types (the default).
+#    enable_relr_relocations: Optional flag. If true, and if |enable_compressed_relocations|
+#      is true, then reduce the size of the native library by enabling RELR relocations.
+#      Note that these are only supported by the Chromium linker, and Android P+.
 template("chrome_common_shared_library") {
   shared_library(target_name) {
     forward_variables_from(invoker,
                            "*",
                            [
                              "enable_compressed_relocations",
+                             "enable_relr_relocations",
                              "export_java_symbols",
                              "use_gnu_hash_style",
                            ])
@@ -1018,7 +1022,12 @@ template("chrome_common_shared_library") {
     # Compress relocations if needed.
     if (defined(invoker.enable_compressed_relocations) &&
         invoker.enable_compressed_relocations) {
-      configs += [ "//build/config/android:lld_pack_relocations" ]
+      if (defined(invoker.enable_relr_relocations) &&
+          invoker.enable_relr_relocations) {
+        configs += [ "//build/config/android:lld_pack_relocations_with_relr" ]
+      } else {
+        configs += [ "//build/config/android:lld_pack_relocations" ]
+      }
     }
 
     if (invoker.use_gnu_hash_style && target_cpu != "mipsel" &&
@@ -1051,6 +1060,7 @@ template("chrome_shared_library") {
     }
 
     enable_compressed_relocations = chromium_linker_supported && use_lld
+    enable_relr_relocations = enable_compressed_relocations
     use_gnu_hash_style = chromium_linker_supported
   }
 }