mkvmuxer: Fix hard-coded data size in EbmlElementSize

EbmlElementSize for char* assumes that the varint encoding of
payload size is always 1-byte. Fix that by using the correct
number of bytes.

mkvinfo of the new gold file: http://pastebin.com/ncR7DqEf

Change-Id: I2d10708de77b2b8089900a8719ee3797dfb6994c

README.libwebm

@@ -75,7 +75,7 @@ The tests rely upon the LIBWEBM_TEST_DATA_PATH environment variable to locate
 test input. The following example demonstrates running the muxer tests from the
 build directory:
 
-$ LIBWEBM_TEST_DATA_PATH=path/to/libwebm/testing/testdata ./muxer_tests
+$ LIBWEBM_TEST_DATA_PATH=path/to/libwebm/testing/testdata ./mkvmuxer_tests
 
 Note: Libwebm Googletest integration was built with googletest from
       https://github.com/google/googletest.git at git revision

mkvmuxer/mkvmuxerutil.cc

@@ -288,7 +288,7 @@ uint64 EbmlElementSize(uint64 type, const char* value) {
   ebml_size += strlen(value);
 
   // Size of Datasize
-  ebml_size++;
+  ebml_size += GetCodedUIntSize(strlen(value));
 
   return ebml_size;
 }

testing/mkvmuxer_tests.cc

@@ -30,6 +30,7 @@ using mkvmuxer::Frame;
 using mkvmuxer::MkvWriter;
 using mkvmuxer::Segment;
 using mkvmuxer::SegmentInfo;
+using mkvmuxer::Tag;
 using mkvmuxer::Track;
 using mkvmuxer::VideoTrack;
 
@@ -996,6 +997,24 @@ TEST_F(MuxerTest, SetPixelWidthPixelHeight) {
                            filename_));
 }
 
+TEST_F(MuxerTest, LongTagString) {
+  EXPECT_TRUE(SegmentInit(false, false, false));
+  segment_.set_estimate_file_duration(false);
+  AddVideoTrack();
+  Tag* const tag = segment_.AddTag();
+  // 160 needs two bytes when varint encoded.
+  const std::string dummy_string(160, '0');
+  tag->add_simple_tag("long_tag", dummy_string.c_str());
+
+  EXPECT_TRUE(segment_.AddFrame(dummy_data_, kFrameLength, kVideoTrackNumber, 0,
+                                false));
+
+  segment_.Finalize();
+  CloseWriter();
+
+  EXPECT_TRUE(CompareFiles(GetTestFilePath("long_tag_string.webm"), filename_));
+}
+
 }  // namespace test
 
 int main(int argc, char* argv[]) {