HasOnlyAsciiChars can return incorrect results. Fixup usages and rename.

src/log.cc

Index: src/log.cc
diff --git a/src/log.cc b/src/log.cc
index 789e12dfe03b5c54e3dca8ecba5d76740d5c0406..e52d0f3969f4d0b0dc15d451cc1f2a08ab5984a2 100644
--- a/src/log.cc
+++ b/src/log.cc
@@ -335,15 +335,6 @@ class Logger::NameBuffer {
 
   void AppendString(String* str) {
     if (str == NULL) return;
-    if (str->HasOnlyAsciiChars()) {
-      int utf8_length = Min(str->length(), kUtf8BufferSize - utf8_pos_);
-      String::WriteToFlat(str,
-                          reinterpret_cast<uint8_t*>(utf8_buffer_ + utf8_pos_),
-                          0,
-                          utf8_length);
-      utf8_pos_ += utf8_length;
-      return;
-    }
     int uc16_length = Min(str->length(), kUtf16BufferSize);
     String::WriteToFlat(str, utf16_buffer, 0, uc16_length);
     int previous = unibrow::Utf16::kNoPreviousCharacter;