Change name of generic Media Router icon file.

This makes the naming convention uniform with the other Media Router icons.

BUG=464222

Review URL: https://codereview.chromium.org/1053323003

Cr-Commit-Position: refs/heads/master@{#324069}

chrome/app/theme/theme_resources.grd

@@ -823,9 +823,9 @@
       </if>
       <!-- Media Router -->
       <if expr="enable_media_router">
-        <structure type="chrome_scaled_image" name="IDR_MEDIA_ROUTER_ICON" file="common/media_router.png" />
         <structure type="chrome_scaled_image" name="IDR_MEDIA_ROUTER_ACTIVE_ICON" file="common/media_router_active.png" />
         <structure type="chrome_scaled_image" name="IDR_MEDIA_ROUTER_ERROR_ICON" file="common/media_router_error.png" />
+        <structure type="chrome_scaled_image" name="IDR_MEDIA_ROUTER_IDLE_ICON" file="common/media_router_idle.png" />
         <structure type="chrome_scaled_image" name="IDR_MEDIA_ROUTER_WARNING_ICON" file="common/media_router_warning.png" />
       </if>
       <if expr="enable_app_list">