Optimize calls to std::string::find() and friends for a single char.

The character literal overload is more efficient.

go/lsc-performance-faster-string-find

Tested:
    TAP for global presubmit queue
    http://test/OCL:347655100:BASE:347631998:1608131503084:adc733c1
PiperOrigin-RevId: 347936227

Author: Googler

app/instance_id/instance_id_desktop_impl.cc

@@ -721,7 +721,7 @@ bool InstanceIdDesktopImpl::FetchServerToken(const char* scope, bool* retry) {
       size_t component_start = 0;
       size_t component_end;
       do {
-        component_end = error.find(":", component_start);
+        component_end = error.find(':', component_start);
         std::string error_component =
             error.substr(component_start, component_end - component_start);
         if (error_component == kPhoneRegistrationError) {

app/src/locale.cc

@@ -69,12 +69,12 @@ std::string GetLocale() {
   // Some of the environment variables have a "." after the name to specify the
   // terminal encoding, e.g.  "en_US.UTF-8", so we want to cut the string on the
   // ".".
-  size_t cut = output.find(".");
+  size_t cut = output.find('.');
   if (cut != std::string::npos) {
     output = output.substr(0, cut);
   }
   // Do the same with a "@" which some locales also have.
-  cut = output.find("@");
+  cut = output.find('@');
   if (cut != std::string::npos) {
     output = output.substr(0, cut);
   }

app/tests/locale_test.cc

@@ -49,7 +49,7 @@ TEST_F(LocaleTest, TestGetLocale) {
   // Make sure this looks like a locale, e.g. has at least 5 characters and
   // contains an underscore.
   EXPECT_GE(loc.size(), 5);
-  EXPECT_NE(loc.find("_"), std::string::npos);
+  EXPECT_NE(loc.find('_'), std::string::npos);
 }
 
 }  // namespace internal

auth/src/desktop/rpcs/error_codes.cc

@@ -225,7 +225,7 @@ AuthError GetAuthErrorCode(const std::string& error,
     // Couldn't find it, check again with just the first word.
     // Sometimes the backend will return the error code along with
     // some explanatory text (see kAuthErrorNoSuchProvider above).
-    size_t space = error.find(" ");
+    size_t space = error.find(' ');
     if (space != std::string::npos) {
       std::string first_word = error.substr(0, space);
       found = find_error(first_word);

storage/src/common/storage_uri_parser.cc

@@ -45,7 +45,7 @@ static const char kBucketPathSeparator[] = "/o/";
 // Strip the trailing slash from the specified string, leaving the string
 // unmodified if it's not found.
 static std::string StripTrailingSlash(const std::string& value) {
-  auto it = value.rfind("/");
+  auto it = value.rfind('/');
   return it == value.length() - 1 ? value.substr(0, it) : value;
 }
 
@@ -82,7 +82,7 @@ bool UriToComponents(const std::string& url, const char* object_type,
     return false;
   }
   std::string full_path(url.substr(scheme.length()));
-  std::string::size_type it = full_path.find("/");
+  std::string::size_type it = full_path.find('/');
   std::string host(full_path);
   if (it != std::string::npos) host = full_path.substr(0, it);
   std::string remaining_path(full_path.substr(host.length()));