Ensure WebView 64_32 target uses 32-bit abi as default.

Chrome will run in 64-bit, but WebView will run in
32-bit mode by default.

Change-Id: Idc240b62a5c18c9109c02853459c073e4c53d851
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536784
Commit-Queue: Ben Mason <benmason@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828936}

android_webview/nonembedded/BUILD.gn

@@ -194,7 +194,6 @@ if (android_64bit_target_cpu) {
     input = "java/AndroidManifest.xml"
     output = trichrome_webview_64_32_android_manifest
     variables = trichrome_jinja_variables + _webview_jinja_variables + [
-                  "use32bitAbi=",
                   "trichrome_version=$trichrome_64_32_version_code",
                   "library=libmonochrome_64.so",
                 ]