Reland "[WebLayer] Enable WebLayer in split on canary"

This is a reland of 7cfe51d0

The build error should be fixed by https://crrev.com/c/2416892

Original change's description:
> [WebLayer] Enable WebLayer in split on canary
>
> This enables WebLayer in a split in WebView bundles in canary. I tested
> on L-Q and ran all instrumentation tests on P with this setup.
>
> Bug: 1105096
> Change-Id: I517bce4109531108a4fb3af64a87aa1f5d2a91bb
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412705
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Clark DuVall <cduvall@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#807252}

Bug: 1105096
Change-Id: I0d179670f0979809866ae278d7a4e6fd7a58dfbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416878Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807995}

weblayer/variables.gni

@@ -2,12 +2,15 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/android/channel.gni")
+
 declare_args() {
   # Include the //weblayer code in WebView implementation APKs.
   webview_includes_weblayer = true
 
   # Whether WebLayer will be included as a DFM.
-  weblayer_in_split = false
+  weblayer_in_split =
+      android_channel == "default" || android_channel == "canary"
 }
 
 weblayer_product_config_java_package = "org.chromium.weblayer_private"