Revert "[parser] Refactor streaming scanner streams."

This reverts commit ce538f70.

Reason for revert: breaks BOM handling (thus breaking Outlook web apps).

Original change's description:
> [parser] Refactor streaming scanner streams.
> 
> Unify, simplify logic, reduce UTF8 specific handling.
> 
> Intend of this is also to have stream views.
> Stream views can be used concurrently by multiple threads, but
> only one thread may fetch new data from the underlying source.
> This together with unified stream view creation is intended to be
> used for parse tasks.
> 
> BUG=v8:6093
> 
> Change-Id: Ied8e93090c506d4735080298f0fdaeed32043915
> Reviewed-on: https://chromium-review.googlesource.com/501789
> Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
> Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
> Reviewed-by: Marja Hölttä <marja@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#45336}

TBR=marja@chromium.org,vogelheim@chromium.org,jochen@chromium.org,wiktorg@google.com
BUG=v8:6093, chromium:724166

Change-Id: I022a23b8052d20d83a640c07b7864c622548bf90
Reviewed-on: https://chromium-review.googlesource.com/508888Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45404}

include/v8.h

@@ -1266,6 +1266,11 @@ class V8_EXPORT ScriptCompiler {
      * length of the data returned. When the data ends, GetMoreData should
      * return 0. Caller takes ownership of the data.
      *
+     * When streaming UTF-8 data, V8 handles multi-byte characters split between
+     * two data chunks, but doesn't handle multi-byte characters split between
+     * more than two data chunks. The embedder can avoid this problem by always
+     * returning at least 2 bytes of data.
+     *
      * If the embedder wants to cancel the streaming, they should make the next
      * GetMoreData call return 0. V8 will interpret it as end of data (and most
      * probably, parsing will fail). The streaming task will return as soon as

test/cctest/parsing/test-scanner-streams.cc

@@ -435,18 +435,6 @@ TEST(CharacterStreams) {
   TestCharacterStreams(buffer, arraysize(buffer) - 1, 576, 3298);
 }
 
-TEST(Uft8MultipleBOMChunks) {
-  const char* chunks = "\xef\xbb\xbf\0\xef\xbb\xbf\0\xef\xbb\xbf\0a\0";
-  const uint16_t unicode[] = {0xFEFF, 0xFEFF, 97};
-  ChunkSource chunk_source(chunks);
-  std::unique_ptr<i::Utf16CharacterStream> stream(i::ScannerStream::For(
-      &chunk_source, v8::ScriptCompiler::StreamedSource::UTF8, nullptr));
-  for (size_t i = 0; i < arraysize(unicode); i++) {
-    CHECK_EQ(unicode[i], stream->Advance());
-  }
-  CHECK_EQ(i::Utf16CharacterStream::kEndOfInput, stream->Advance());
-}
-
 // Regression test for crbug.com/651333. Read invalid utf-8.
 TEST(Regress651333) {
   const uint8_t bytes[] =