Add include_examples GYP variable.

To make it possible to exclude the examples when running
GYP on all.gyp.
The webrtc_examples.gyp already has an OS=="android" condition
inside it, so there's no need to check that before including it.

BUG=webrtc:4242

Review URL: https://codereview.webrtc.org/1196623006

Cr-Commit-Position: refs/heads/master@{#9556}

all.gyp

@@ -8,6 +8,7 @@
 
 {
   'variables': {
+    'include_examples%': 1,
     'include_tests%': 1,
     'webrtc_root_additional_dependencies': [],
   },
@@ -18,12 +19,12 @@
       'dependencies': [
         'webrtc/webrtc.gyp:*',
         'talk/libjingle.gyp:*',
-        'talk/libjingle_examples.gyp:*',
         '<@(webrtc_root_additional_dependencies)',
       ],
       'conditions': [
-        ['OS=="android"', {
+        ['include_examples==1', {
           'dependencies': [
+            'talk/libjingle_examples.gyp:*',
             'webrtc/webrtc_examples.gyp:*',
           ],
         }],