Disable plugin installation on iOS

Plugin installation is not supported on iOS, so the code shouldn't
be compiled.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/14826004

git-svn-id: http://src.chromium.org/svn/trunk/src/build@198412 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -549,7 +549,7 @@
           'chromium_win_pch%': 1
         }],
 
-        ['use_aura==1 or chromeos==1 or OS=="android"', {
+        ['use_aura==1 or chromeos==1 or OS=="android" or OS=="ios"', {
           'enable_plugin_installation%': 0,
         }, {
           'enable_plugin_installation%': 1,