[OsSettings] Add row getter to Network Summary and use in InternetPage

Better encapsulate network-summary by preventing InternetPage from
directly accessing its DOM. Adds getNetworkRow public getter method
and uses it in InternetPage.

Change-Id: I8a42c5e13db9476b2bc600e3b6e0d97be3390c6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341757Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Daniel Classon <dclasson@google.com>
Cr-Commit-Position: refs/heads/master@{#795692}

chrome/browser/resources/settings/chromeos/internet_page/internet_page.js

@@ -209,8 +209,8 @@ Polymer({
         element = subPage.$$('#networkList');
       }
     } else if (this.detailType_ !== undefined) {
-      const oncType = OncMojo.getNetworkTypeString(this.detailType_);
-      const rowForDetailType = this.$$('network-summary').$$(`#${oncType}`);
+      const rowForDetailType =
+          this.$$('network-summary').getNetworkRow(this.detailType_);
 
       // Note: It is possible that the row is no longer present in the DOM
       // (e.g., when a Cellular dongle is unplugged or when Instant Tethering

chrome/browser/resources/settings/chromeos/internet_page/network_summary.js

@@ -125,6 +125,17 @@ Polymer({
     this.getNetworkLists_();
   },
 
+  /*
+   * Returns the network-summary-item element corresponding to the
+   * |networkType|.
+   * @param {!chromeos.networkConfig.mojom.NetworkType} networkType
+   * @return {?NetworkSummaryItemElement}
+   */
+  getNetworkRow(networkType) {
+    const networkTypeString = OncMojo.getNetworkTypeString(networkType);
+    return this.$$(`#${networkTypeString}`);
+  },
+
   /**
    * Requests the list of device states and network states from Chrome.
    * Updates deviceStates, activeNetworkStates, and networkStateLists once the