Add *.cer application/x-x509-ca-cert MIME mapping

This is required for selecting and installing Android certificate files
while testing ARC++ on Chrome OS. (See b/133848333)

BUG=b:133848333
TEST=net_unittests

Change-Id: I8e7a45cbd2436e9a2c013dd04a55df6b6a06623c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1637012
Commit-Queue: Satoshi Niwa <niwa@chromium.org>
Auto-Submit: Satoshi Niwa <niwa@chromium.org>
Reviewed-by: Asanka Herath <asanka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665042}

net/base/mime_util.cc

@@ -194,6 +194,7 @@ static const MimeInfo kSecondaryMappings[] = {
     {"application/x-mpegurl", "m3u8"},
     {"application/x-shockwave-flash", "swf,swl"},
     {"application/x-tar", "tar"},
+    {"application/x-x509-ca-cert", "cer,crt"},
     {"application/zip", "zip"},
     {"audio/mpeg", "mp3"},
     // This is the platform mapping on recent versions of Windows 10.

net/base/mime_util_unittest.cc

@@ -37,6 +37,8 @@ TEST(MimeUtilTest, ExtensionTest) {
     // These are test cases for testing platform mime types on Chrome OS.
     {FILE_PATH_LITERAL("epub"), "application/epub+zip", true},
     {FILE_PATH_LITERAL("apk"), "application/vnd.android.package-archive", true},
+    {FILE_PATH_LITERAL("cer"), "application/x-x509-ca-cert", true},
+    {FILE_PATH_LITERAL("crt"), "application/x-x509-ca-cert", true},
     {FILE_PATH_LITERAL("zip"), "application/zip", true},
     {FILE_PATH_LITERAL("ics"), "text/calendar", true},
 #endif