Cherry pick underflow fix.

BUG=583607

Review URL: https://codereview.chromium.org/1662313002

Cr-Commit-Position: refs/heads/master@{#373736}
void BrotliSetCustomDictionary(
    size_t size, const uint8_t* dict, BrotliState* s) {
  s->custom_dict = dict;
  s->custom_dict_size = (int) size;
}

void BrotliSetCustomDictionary(
    size_t size, const uint8_t* dict, BrotliState* s) {
  s->custom_dict = dict;
  s->custom_dict_size = (int) size;
}
