Add marker bit to bool-coded partition start

Adds a marker bit to allow distinguishing the frame header from its residual
data.

Change-Id: Id75d47acc9e5a97007e4690c4f8748a4ce63e641

test/vp9_boolcoder_test.cc

@@ -68,6 +68,9 @@ TEST(VP9, TestBitIO) {
 
         vp9_stop_encode(&bw);
 
+        // First bit should be zero
+        GTEST_ASSERT_EQ(bw_buffer[0] & 0x80, 0);
+
         vp9_reader br;
         vp9_reader_init(&br, bw_buffer, buffer_size);
         bit_rnd.Reset(random_seed);