Revert "Fix //ios/web_view/tools/build.py to reflect new build flags."

This reverts commit a717720f.

Reason for revert: Speculative revert to fix net_unittests failure for Mac bots.

Original change's description:
> Fix //ios/web_view/tools/build.py to reflect new build flags.
> 
> Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
> Change-Id: I79b509fc63a011a8dcdfcf040ef694fc4a2170f8
> Reviewed-on: https://chromium-review.googlesource.com/1092074
> Reviewed-by: Eugene But <eugenebut@chromium.org>
> Commit-Queue: John Wu <jzw@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#565691}

TBR=eugenebut@chromium.org,jzw@chromium.org

Change-Id: Ia5257e274d77be6f3582d972f0d56cd034a6f83b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Reviewed-on: https://chromium-review.googlesource.com/1093479Reviewed-by: Samuel Huang <huangs@chromium.org>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565734}

ios/web_view/tools/build.py

@@ -189,8 +189,8 @@ def main():
                       help='Additional gn args to pass through to ninja.')
   parser.add_argument('--include_cronet', action='store_true',
                       help='Combines Cronet and ChromeWebView as 1 framework.')
-  parser.add_argument('--enable_sync', action='store_true',
-                      help='Enables public API for sync.')
+  parser.add_argument('--enable_signin', action='store_true',
+                      help='Enables public API for signin.')
   parser.add_argument('--enable_autofill', action='store_true',
                       help='Enables public API for autofill.')
   build_configs = ['Debug', 'Release']
@@ -224,10 +224,10 @@ def main():
     output_name = 'CronetChromeWebView'
   else:
     extra_gn_options += 'ios_web_view_include_cronet=false '
-  if options.enable_sync:
-    extra_gn_options += 'ios_web_view_enable_sync=true '
+  if options.enable_signin:
+    extra_gn_options += 'ios_web_view_enable_signin=true '
   else:
-    extra_gn_options += 'ios_web_view_enable_sync=false '
+    extra_gn_options += 'ios_web_view_enable_signin=false '
   if options.enable_autofill:
     extra_gn_options += 'ios_web_view_enable_autofill=true '
   else: