Use auto& instead of auto in the range loop

There are a couple reasons for this change:
1. Avoid copying 2-pointer struct.
2. If any of the pointers in the struct gets replaced by CheckedPtr with
   a non-trivial constructor, the compiler will complain.

See go/miracleptr for more details on the experimental CheckedPtr project.

Bug: 1080832
Change-Id: Ic2c6c1cc37601a8a22cb0bb2177547ea614fbcea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2434021
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811476}

base/profiler/stack_sampling_profiler_test_util.cc

@@ -372,7 +372,7 @@ void ExpectStackDoesNotContain(
 
   std::set<FunctionAddressRange, FunctionAddressRangeCompare> seen_functions;
   for (const auto& frame : stack) {
-    for (const auto function : functions) {
+    for (const auto& function : functions) {
       if (frame.instruction_pointer >=
               reinterpret_cast<uintptr_t>(function.start) &&
           frame.instruction_pointer <=
@@ -382,7 +382,7 @@ void ExpectStackDoesNotContain(
     }
   }
 
-  for (const auto function : seen_functions) {
+  for (const auto& function : seen_functions) {
     ADD_FAILURE() << "Function at " << function.start
                   << " was unexpectedly found in stack:\n"
                   << FormatSampleForDiagnosticOutput(stack);