Issue mozilla-mobile#2805: Add WebBackForwardList.toList.

This code is adapted from Severin's WIP:
severinrudie@0f4f432

app/src/main/java/org/mozilla/tv/firefox/ext/WebBackForwardList.kt

@@ -0,0 +1,17 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.tv.firefox.ext
+
+import android.webkit.WebBackForwardList
+import android.webkit.WebHistoryItem
+
+fun WebBackForwardList.toList(): List<WebHistoryItem> {
+    val size = this.size
+    val outputList = mutableListOf<WebHistoryItem>()
+    for (i in 0 until size) {
+        outputList.add(getItemAtIndex(i))
+    }
+    return outputList
+}

app/src/testSystem/java/org/mozilla/tv/firefox/ext/WebBackForwardListKtTest.kt

@@ -0,0 +1,30 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.tv.firefox.ext
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import org.mozilla.tv.firefox.helpers.MockWebHistoryItem
+import org.mozilla.tv.firefox.helpers.toMockWebBackForwardList
+
+class WebBackForwardListKtTest {
+
+    @Test
+    fun `WHEN a WebBackForwardList is converted to a List THEN the original URLs are all in order and unmodified`() {
+        val expected = listOf(
+            "https://google.com",
+            "https://mozilla.org",
+            "https://facebook.com"
+        )
+
+        // This test relies on our MockWeb* impls having no bugs but we can't do any better.
+        val webBackForwardList = expected.map { MockWebHistoryItem(mockOriginalUrl = it) }
+            .toMockWebBackForwardList()
+
+        val actual = webBackForwardList.toList()
+            .map { it.originalUrl }
+        assertEquals(expected, actual)
+    }
+}