Fix WebP sniff pattern

The current pattern wouldn't be able to sniff the 'lossless' and
'extended' WebP formats.

Adjust the pattern to match the one described by [1].

[1] https://mimesniff.spec.whatwg.org/#matching-an-image-type-pattern

Bug: 889420
Change-Id: I4b5a73d4d9477502d58df7460b9494e6540a59f3
Reviewed-on: https://chromium-review.googlesource.com/1248782Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#595273}

net/base/mime_sniffer.cc

@@ -152,7 +152,7 @@ static const MagicNumber kMagicNumbers[] = {
   MAGIC_NUMBER("image/tiff", "II*"),
   MAGIC_NUMBER("image/tiff", "MM\x00*"),
   MAGIC_NUMBER("audio/mpeg", "ID3"),
-  MAGIC_NUMBER("image/webp", "RIFF....WEBPVP8 "),
+  MAGIC_NUMBER("image/webp", "RIFF....WEBPVP"),
   MAGIC_NUMBER("video/webm", "\x1A\x45\xDF\xA3"),
   MAGIC_NUMBER("application/zip", "PK\x03\x04"),
   MAGIC_NUMBER("application/x-rar-compressed", "Rar!\x1A\x07\x00"),

net/base/mime_sniffer_unittest.cc

@@ -458,6 +458,27 @@ TEST(MimeSnifferTest, AudioVideoTest) {
   mime_type.clear();
 }
 
+TEST(MimeSnifferTest, ImageTest) {
+  std::string mime_type;
+  const char kWebPSimpleFormat[] = "RIFF\xee\x81\x00\x00WEBPVP8 ";
+  EXPECT_TRUE(SniffMimeTypeFromLocalData(
+      kWebPSimpleFormat, sizeof(kWebPSimpleFormat) - 1, &mime_type));
+  EXPECT_EQ("image/webp", mime_type);
+  mime_type.clear();
+
+  const char kWebPLosslessFormat[] = "RIFF\xee\x81\x00\x00WEBPVP8L";
+  EXPECT_TRUE(SniffMimeTypeFromLocalData(
+      kWebPLosslessFormat, sizeof(kWebPLosslessFormat) - 1, &mime_type));
+  EXPECT_EQ("image/webp", mime_type);
+  mime_type.clear();
+
+  const char kWebPExtendedFormat[] = "RIFF\xee\x81\x00\x00WEBPVP8X";
+  EXPECT_TRUE(SniffMimeTypeFromLocalData(
+      kWebPExtendedFormat, sizeof(kWebPExtendedFormat) - 1, &mime_type));
+  EXPECT_EQ("image/webp", mime_type);
+  mime_type.clear();
+}
+
 // The tests need char parameters, but the ranges to test include 0xFF, and some
 // platforms have signed chars and are noisy about it. Using an int parameter
 // and casting it to char inside the test case solves both these problems.