[WebLayerGPaytest] Test the integration between WebLayer and Payments

Changes:
* Adds a WebLayer junit target: weblayer_junit_tests.
* Adds an integration tests for payment request service in WebLayer.

Bug: 1170916
Change-Id: I2bf2ee729384d4b30806c2977424e96f3255d79d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2650657
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Haiyang Pan <hypan@google.com>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#849857}

diff --git a/BUILD.gn b/BUILD.gn
index b6af235..5e93c67 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -217,7 +217,10 @@
       "//weblayer/test:weblayer_unittests",
     ]
     if (is_android) {
-      deps += [ "//weblayer/browser/android/javatests:weblayer_instrumentation_test_apk" ]
+      deps += [
+        "//weblayer/browser/android/javatests:weblayer_instrumentation_test_apk",
+        "//weblayer/browser/java:weblayer_junit_tests",
+      ]
     }
   }