upkeep: Use char overload of r?find() where possible

Change-Id: I34518ddc604f1d8925b64a057d1dbb65a749121e
diff --git a/src/trace_processor/trace_processor_shell.cc b/src/trace_processor/trace_processor_shell.cc
index c2755a9..144810e 100644
--- a/src/trace_processor/trace_processor_shell.cc
+++ b/src/trace_processor/trace_processor_shell.cc
@@ -240,7 +240,7 @@
 }
 
 util::Status ExportTraceToDatabase(const std::string& output_name) {
-  PERFETTO_CHECK(output_name.find("'") == std::string::npos);
+  PERFETTO_CHECK(output_name.find('\'') == std::string::npos);
   {
     base::ScopedFile fd(base::OpenFile(output_name, O_CREAT | O_RDWR, 0600));
     if (!fd)
@@ -265,7 +265,7 @@
       "SELECT name FROM sqlite_master WHERE type='table'");
   for (uint32_t rows = 0; tables_it.Next(); rows++) {
     std::string table_name = tables_it.Get(0).string_value;
-    PERFETTO_CHECK(table_name.find("'") == std::string::npos);
+    PERFETTO_CHECK(table_name.find('\'') == std::string::npos);
     std::string export_sql = "CREATE TABLE perfetto_export." + table_name +
                              " AS SELECT * FROM " + table_name;
 
@@ -957,7 +957,7 @@
     const std::string& metric_or_path = metrics[i];
 
     // If there is no extension, we assume it is a builtin metric.
-    auto ext_idx = metric_or_path.rfind(".");
+    auto ext_idx = metric_or_path.rfind('.');
     if (ext_idx == std::string::npos)
       continue;