Optimize BitStreamReader, speed up EV Whitelist loading

components/packed_ct_ev_whitelist/bit_stream_reader.cc

Index: components/packed_ct_ev_whitelist/bit_stream_reader.cc
diff --git a/components/packed_ct_ev_whitelist/bit_stream_reader.cc b/components/packed_ct_ev_whitelist/bit_stream_reader.cc
index e4f6cc7b64a8896055bc56dd85f272d891b7f745..94ef00904bbd9ae79758a6f24552022be90184bb 100644
--- a/components/packed_ct_ev_whitelist/bit_stream_reader.cc
+++ b/components/packed_ct_ev_whitelist/bit_stream_reader.cc
@@ -34,8 +34,13 @@ bool BitStreamReader::ReadBits(uint8_t num_bits, uint64_t* out) {
     return false;
 
   *out = 0;
-  for (uint8_t i = 0; i < num_bits; ++i)
-    (*out) |= (static_cast<uint64_t>(ReadBit()) << (num_bits - (i + 1)));
+
+  for (; num_bits && (current_bit_ != 7); --num_bits)
+    (*out) |= (static_cast<uint64_t>(ReadBit()) << (num_bits - 1));
+  for (; num_bits / 8; num_bits -= 8)
+    (*out) |= (static_cast<uint64_t>(ReadByte()) << (num_bits - 8));
+  for (; num_bits; --num_bits)
+    (*out) |= (static_cast<uint64_t>(ReadBit()) << (num_bits - 1));
 
   return true;
 }
@@ -61,5 +66,13 @@ uint8_t BitStreamReader::ReadBit() {
   return res;
 }
 
+uint8_t BitStreamReader::ReadByte() {
+  DCHECK_GT(BitsLeft(), 7u);
+  DCHECK_EQ(current_bit_, 7);
+
+  return source_.data()[current_byte_++];
+
+}
+
 }  // namespace internal
 }  // namespace packed_ct_ev_whitelist