[asm.js] Fix semicolon insertion in presence of comments.

This makes sure we properly recognize a newline character as part of
semicolon insertion, even if the newline appears after a CPP-style
single line comment. The same applies for newlines within C-style multi
line comments.

R=clemensh@chromium.org
TEST=mjsunit/asm/regress-913822
BUG=chromium:913822

Change-Id: I64f098d7e386dea7b7fb6c233c1625425e36bde0
Reviewed-on: https://chromium-review.googlesource.com/c/1373551Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58189}

src/asmjs/asm-scanner.cc

@@ -349,6 +349,9 @@ bool AsmJsScanner::ConsumeCComment() {
         return true;
       }
     }
+    if (ch == '\n') {
+      preceded_by_newline_ = true;
+    }
     if (ch == kEndOfInput) {
       return false;
     }
@@ -358,7 +361,11 @@ bool AsmJsScanner::ConsumeCComment() {
 void AsmJsScanner::ConsumeCPPComment() {
   for (;;) {
     uc32 ch = stream_->Advance();
-    if (ch == '\n' || ch == kEndOfInput) {
+    if (ch == '\n') {
+      preceded_by_newline_ = true;
+      return;
+    }
+    if (ch == kEndOfInput) {
       return;
     }
   }

test/mjsunit/asm/regress-913822.js

+// Copyright 2018 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+(function TestNewlineInCPPComment() {
+  function Module() {
+    "use asm" // Crash by comment!
+    function f() {}
+    return f
+  }
+  Module();
+  assertTrue(%IsAsmWasmCode(Module));
+})();
+
+(function TestNewlineInCComment() {
+  function Module() {
+    "use asm" /* Crash by
+    comment! */ function f() {}
+    return f
+  }
+  Module();
+  assertTrue(%IsAsmWasmCode(Module));
+})();