Avoid implicit narrowing std::string::operator= in AsmJsScanner

Change-Id: Ic11e2c6a7b08b7f470f8f1649f37c48cc53eeee6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1507675Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60089}

src/asmjs/asm-scanner.cc

@@ -271,7 +271,7 @@ void AsmJsScanner::ConsumeIdentifier(uc32 ch) {
 
 void AsmJsScanner::ConsumeNumber(uc32 ch) {
   std::string number;
-  number = ch;
+  number.assign(1, ch);
   bool has_dot = ch == '.';
   bool has_prefix = false;
   for (;;) {