Create a Promise class to simplify dealing with async results.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index 7eddca0178b1c5a3ad416ce67fac1aedf8656aa3..99557703f4e60ef5e463ad97979abc606cdbb79d 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -2157,6 +2157,7 @@ if (is_android) {
       "android/java/src/org/chromium/base/PerfTraceEvent.java",
       "android/java/src/org/chromium/base/PowerMonitor.java",
       "android/java/src/org/chromium/base/PowerStatusReceiver.java",
+      "android/java/src/org/chromium/base/Promise.java",
       "android/java/src/org/chromium/base/ResourceExtractor.java",
       "android/java/src/org/chromium/base/SecureRandomInitializer.java",
       "android/java/src/org/chromium/base/StreamUtil.java",
@@ -2273,6 +2274,7 @@ if (is_android) {
     java_files = [
       "android/junit/src/org/chromium/base/BaseChromiumApplicationTest.java",
       "android/junit/src/org/chromium/base/LogTest.java",
+      "android/junit/src/org/chromium/base/PromiseTest.java",
       "test/android/junit/src/org/chromium/base/test/util/DisableIfTest.java",
       "test/android/junit/src/org/chromium/base/test/util/MinAndroidSdkLevelSkipCheckTest.java",
       "test/android/junit/src/org/chromium/base/test/util/RestrictionSkipCheckTest.java",