Add functions for DER parsing a BIT STRING.

net/der/parser.h

Index: net/der/parser.h
diff --git a/net/der/parser.h b/net/der/parser.h
index 901413363e3835c5bc16a6cf4038109184038fca..28b495d011212417b7689bf6ee8a381a332ce25e 100644
--- a/net/der/parser.h
+++ b/net/der/parser.h
@@ -145,8 +145,27 @@ class NET_EXPORT Parser {
   // current value. Note that DER-encoded integers are arbitrary precision,
   // so this method will fail for valid input that represents an integer
   // outside the range of an int64.
+  //
+  // Note that on failure the Parser is left in an undefined state (the
+  // input may or may not have been advanced).
   bool ReadUint64(uint64_t* out) WARN_UNUSED_RESULT;
 
+  // Reads a BIT STRING. On success |bytes| is set to the octet string
+  // containing the bits, and |unused_bits| is set to the number of
+  // bits (in the range 0-7) that are unused.
+  //
+  // The bits are ordered within each octet of |bytes| from most to
+  // least significant, as in the DER encoding.
+  //
+  // Note that on failure the Parser is left in an undefined state (the
+  // input may or may not have been advanced).
+  bool ReadBitString(Input* bytes, uint8_t* unused_bits) WARN_UNUSED_RESULT;
+
+  // Same as ReadBitString() except it will only succeed when the number of
+  // bits in the bit string is a multiple of 8. In other words, there can be
+  // no unused bits in the octet string |*bytes|.
+  bool ReadBitStringNoUnusedBits(Input* bytes) WARN_UNUSED_RESULT;
+
   // Lower level methods. The previous methods couple reading data from the
   // input with advancing the Parser's internal pointer to the next TLV; these
   // lower level methods decouple those two steps into methods that read from