Make payment manifest download/parse cross-platform

Reason for the refactor is an upcoming change to download/parse the
payment manifest on desktop in addition to Android.

1) This patch moves downloader and parser utility into cross-platform
   directory //components/payments/content.

2) The cross-platform PaymentManifestParserHost has been separated from
   its Android wrapper PaymentManifestParserAndroid.

3) GN build target renames:
   a) Payments mojom targets have been renamed to include the string
      "mojom" for clarity.
   b) The main target of "//components/payments/content" has been
      renamed to "content" for brevity.
   c) The "//components/payments/content:payment_validators" target has
      been renamed to "//components/payments/content:utils", because it
      now contains manifest downloader and parser.

Manifest download is currently used only on Android after enabling the
chrome://flags/#android-payment-apps flag.

BUG=683329

Review-Url: https://codereview.chromium.org/2759283002
Cr-Commit-Position: refs/heads/master@{#460141}
diff --git a/content/browser/BUILD.gn b/content/browser/BUILD.gn
index 69007306..34cede5d 100644
--- a/content/browser/BUILD.gn
+++ b/content/browser/BUILD.gn
@@ -44,7 +44,7 @@
     "//components/leveldb:lib",
     "//components/link_header_util",
     "//components/mime_util",
-    "//components/payments/content:payment_app",
+    "//components/payments/content:mojom_payment_app",
     "//components/rappor",
     "//components/tracing",
     "//components/tracing:startup_tracing",