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 3063fa2ea657edd2efe96e74e7315454c282062c..88f4dfb5526be3e57ff2efad3b5f07dd3f88fa90 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -2252,6 +2252,7 @@ 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",
@@ -2370,6 +2371,7 @@ 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",