crosswalk-18: Make |enable_webvr==0| work.

Move CardboardVRDevice.java to a different directory so that it is not
built by the `content_java` target even if `enable_webvr==0`.

This is a prerequisite for building Crosswalk with `enable_webvr=0` set
by default in the crosswalk-18 branch.

BUG=XWALK-6746

Author: Raphael Kubo da Costa

content/content.gyp

@@ -465,6 +465,11 @@
             'has_java_resources': 1,
             'R_package': 'org.chromium.content',
             'R_package_relpath': 'org/chromium/content',
+            'conditions': [
+               ['enable_webvr==1', {
+                 'additional_src_dirs': ['../content/public/android/java-optional'],
+               }],
+             ],
           },
           'includes': [ '../build/java.gypi' ],
         },
@@ -611,7 +616,7 @@
           'conditions': [
             ['enable_webvr==1', {
               'sources': [
-                'public/android/java/src/org/chromium/content/browser/input/CardboardVRDevice.java',
+                'public/android/java-optional/src/org/chromium/content/browser/input/CardboardVRDevice.java',
               ],
               'dependencies': [
                 '../third_party/cardboard-java/cardboard.gyp:cardboard_jar',