Change Picture: Add property to disable motion stills.

This adds a property to cr_camera that allows video mode to be
disabled by hiding camera mode button. It is also making
video mode disabled by default.

Bug: 769140
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I232e95dae75502b4242a807dcfe4e6d668120312
Reviewed-on: https://chromium-review.googlesource.com/745121Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512862}

ui/webui/resources/cr_elements/chromeos/cr_picture/cr_camera.html

@@ -151,7 +151,7 @@
       <div>
         <button is="paper-icon-button-light" id="switchMode" tabindex="2"
             title="[[switchModeLabel]]" on-tap="onTapSwitchMode_"
-            disabled="[[!cameraOnline_]]">
+            disabled="[[!cameraOnline_]]" hidden="[[!videomodeEnabled]]">
         </button>
       </div>
     </div>

ui/webui/resources/cr_elements/chromeos/cr_picture/cr_camera.js

@@ -37,6 +37,12 @@ Polymer({
     takePhotoLabel: String,
     switchModeLabel: String,
 
+    /** True if video mode is enabled. */
+    videomodeEnabled: {
+      type: Boolean,
+      value: false,
+    },
+
     /**
      * True if currently in video mode.
      * @private {boolean}