[cleanup] Remove DCHECK from third_party

DCHECKs (defined in src/logging.h) should not be used in third_party
code. This CL removes the only such use.

R=marja@chromium.org

Bug: v8:7754, v8:7965
Change-Id: I44196bd2d85e459e27c6a897f039f741edf1e2e1
Reviewed-on: https://chromium-review.googlesource.com/1158684Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54967}

src/third_party/utf8-decoder/utf8-decoder.h

@@ -67,7 +67,6 @@ static inline void Decode(uint8_t byte, State* state, uint32_t* buffer) {
       0,  0,  36, 36, 0,  0,  0,  0,  0,  0, 0,  0,   // 4-byte mid/high = 96
   };
 
-  DCHECK_NE(*state, State::kReject);
   uint8_t type = transitions[byte];
   *state = static_cast<State>(states[*state + type]);
   *buffer = (*buffer << 6) | (byte & (0x7F >> (type >> 1)));

tools/generate-header-include-checks.py

@@ -41,7 +41,6 @@ AUTO_EXCLUDE = [
   'src/parsing/preparser.h',
   'src/regexp/jsregexp.h',
   'src/snapshot/object-deserializer.h',
-  'src/third_party/utf8-decoder/utf8-decoder.h',
   'src/transitions.h',
 ]
 AUTO_EXCLUDE_PATTERNS = [