[aw] Test loadUrl with ImmutableMap

Change testCanInjectHeaders to pass an ImmutableMap to loadUrl instead
of a mutable map. Some older WebView versions would crash if an
immutable map was passed. This test now prevents this to regress in the
future.

R=ntfschr

Test: run_webview_instrumentation_test_apk -f *testCanInjectHeaders*
Change-Id: Ie74893361088dec46f0470ba934b1d185dc0f9fb
Fixed: 1032049
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2449473
Auto-Submit: Laís Minchillo <laisminchillo@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815128}

android_webview/javatests/src/org/chromium/android_webview/test/AwContentsTest.java

@@ -25,6 +25,8 @@ import androidx.test.filters.LargeTest;
 import androidx.test.filters.MediumTest;
 import androidx.test.filters.SmallTest;
 
+import com.google.common.collect.ImmutableMap;
+
 import org.junit.Assert;
 import org.junit.Rule;
 import org.junit.Test;
@@ -538,10 +540,8 @@ public class AwContentsTest {
 
             url = testServer.getURL("/echoheader?Referer");
 
-            extraHeaders.clear();
-            extraHeaders.put("Referer", "http://www.example.com/");
-            mActivityTestRule.loadUrlSync(
-                    awContents, mContentsClient.getOnPageFinishedHelper(), url, extraHeaders);
+            mActivityTestRule.loadUrlSync(awContents, mContentsClient.getOnPageFinishedHelper(),
+                    url, ImmutableMap.of("Referer", "http://www.example.com/"));
 
             String referer = mActivityTestRule.getJavaScriptResultBodyTextContent(
                     awContents, mContentsClient);