Add *.weba audio/webm MIME mapping.

Windows 10 now maps audio/webm to *.weba by default. This CL fixes a
test that change broken (MimeUtilTest.GetPreferredExtensionForMimeType),
and adds a secondary mapping for audio/webm to weba.

Bug: 923502
Change-Id: Ia2e3344c5bf270a6bcfe5154ce8a4e28285821fc
Reviewed-on: https://chromium-review.googlesource.com/c/1436939Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626104}

net/base/mime_util.cc

@@ -196,6 +196,8 @@ static const MimeInfo kSecondaryMappings[] = {
     {"application/x-tar", "tar"},
     {"application/zip", "zip"},
     {"audio/mpeg", "mp3"},
+    // This is the platform mapping on recent versions of Windows 10.
+    {"audio/webm", "weba"},
     {"image/bmp", "bmp"},
     {"image/jpeg", "jfif,pjpeg,pjp"},
     {"image/svg+xml", "svg,svgz"},

net/base/mime_util_unittest.cc

@@ -32,6 +32,7 @@ TEST(MimeUtilTest, ExtensionTest) {
     {FILE_PATH_LITERAL("json"), "application/json", true},
     {FILE_PATH_LITERAL("js"), "text/javascript", true},
     {FILE_PATH_LITERAL("webm"), "video/webm", true},
+    {FILE_PATH_LITERAL("weba"), "audio/webm", true},
 #if defined(OS_CHROMEOS)
     // These are test cases for testing platform mime types on Chrome OS.
     {FILE_PATH_LITERAL("epub"), "application/epub+zip", true},
@@ -70,7 +71,6 @@ TEST(MimeUtilTest, GetPreferredExtensionForMimeType) {
       {"application/wasm", FILE_PATH_LITERAL("wasm")},      // Primary
       {"application/javascript", FILE_PATH_LITERAL("js")},  // Secondary
       {"text/javascript", FILE_PATH_LITERAL("js")},         // Primary
-      {"audio/webm", FILE_PATH_LITERAL("webm")},            // Primary
       {"video/webm", FILE_PATH_LITERAL("webm")},            // Primary
   };