Define proper build args for Trichrome and fix cert.

Make the Trichrome library package name and certificate digest into GN
args so that anyone building Trichrome can easily override them.

Change the default certificate digest to be the one for the upstream dev
certificate (the one used by default if you check out and build chromium
with no further configuration) so that upstream builds actually function
out of the box, but make the default a GN variable so that downstream
can override it back to the downstream dev certificate (the old value
here).

Bug: 887078
Change-Id: I988e304e7166313ca196793c595db5f00a41cce2
Reviewed-on: https://chromium-review.googlesource.com/c/1393524Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619501}

chrome/android/trichrome.gni

@@ -8,9 +8,20 @@ import("//build/config/android/config.gni")
 import("//build/config/android/rules.gni")
 import("//chrome/android/chrome_public_apk_tmpl.gni")
 
-trichrome_library_package = "org.chromium.trichromelibrary"
-trichrome_certdigest =
-    "1975b2f17177bc89a5dff31f9e64a6cae281a53dc1d1d59b1d147fe1c82afa00"
+if (!defined(default_trichrome_certdigest)) {
+  default_trichrome_certdigest =
+      "32a2fc74d731105859e5a85df16d95f102d85b22099b8064c5d8915c61dad1e0"
+}
+
+declare_args() {
+  # The package name for the Trichrome static shared library on Android.
+  trichrome_library_package = "org.chromium.trichromelibrary"
+
+  # The SHA256 certificate digest for the Trichrome static shared library on
+  # Android. You can use "apksigner verify --print-certs" on the signed APK to
+  # calculate the correct digest.
+  trichrome_certdigest = default_trichrome_certdigest
+}
 
 trichrome_jinja_variables = [
   "min_sdk_version=28",