webview: update AOSP build options

Mention fieldtrial_testing_like_official_build to ensure builds don't
get the test field trial config.

Change-Id: I3f664bcf0c81d6d6dde33a4830e13f0dc11e15b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2269747
Auto-Submit: Richard Coles <torne@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783052}

android_webview/docs/aosp-system-integration.md

@@ -188,6 +188,10 @@ target_cpu = "arm64"       # or "arm", "x86", or "x64"; see below
 is_debug = false
 is_official_build = true
 
+# Use the default production settings for field trials, instead of the testing
+# defaults.
+fieldtrial_testing_like_official_build = true
+
 # WebView's efficient native library loading mechanism is not compatible with
 # component builds of Chromium.
 is_component_build = false