Really fix building android webview on mac.

The refactoring in the final patchset landed as r269766 got the
condition backwards (the pre-refactoring version worked), so we were
accidentally excluding flag only on *target* builds, instead of host
builds as intended (the target compiler supports this flag). Fix it.

BUG=321833
R=primiano@chromium.org

Review URL: https://codereview.chromium.org/296273002

git-svn-id: http://src.chromium.org/svn/trunk/src/build@272511 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -5231,7 +5231,7 @@
     ['gcc_version>=48 and android_webview_build==1 and host_os=="mac"', {
       'target_defaults': {
         'target_conditions': [
-          ['_toolset=="target"', {
+          ['_toolset=="host"', {
             'cflags!': [
               '-Wno-unused-local-typedefs',
             ],