ui: use a StringPiece instead of std::string

The strings stored in `tokens` all have static storage duration, so
there's no reason to put them in `std::string`s.

AX-Relnotes: n/a.

Bug: None
Change-Id: I9b6bc9e847e5c27d68707a4f33d37b225ded34fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2457589Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817317}

ui/accessibility/ax_mode.cc

@@ -6,6 +6,7 @@
 
 #include <vector>
 
+#include "base/strings/string_piece.h"
 #include "base/strings/string_util.h"
 
 namespace ui {
@@ -15,13 +16,13 @@ std::ostream& operator<<(std::ostream& stream, const AXMode& mode) {
 }
 
 std::string AXMode::ToString() const {
-  std::vector<std::string> tokens;
+  std::vector<base::StringPiece> tokens;
 
   // Written as a loop with a switch so that this crashes if a new
   // mode flag is added without adding support for logging it.
   for (uint32_t mode_flag = AXMode::kFirstModeFlag;
        mode_flag <= AXMode::kLastModeFlag; mode_flag = mode_flag << 1) {
-    const char* flag_name = nullptr;
+    base::StringPiece flag_name;
     switch (mode_flag) {
       case AXMode::kNativeAPIs:
         flag_name = "kNativeAPIs";
@@ -46,7 +47,7 @@ std::string AXMode::ToString() const {
         break;
     }
 
-    DCHECK(flag_name);
+    DCHECK(!flag_name.empty());
 
     if (has_mode(mode_flag))
       tokens.push_back(flag_name);