[WebLayer] Fix weblayer_browsertests failing with ASan

the backing string was getting destroyed too early.

base: :FeatureList::SplitFeatureListString takes in a StringPiece, and
Change-Id: Id054a40451031bc41cbae3d4caa95f25568d0011
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1887830Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710436}

weblayer/app/content_main_delegate_impl.cc

@@ -66,14 +66,18 @@ void DisableFeaturesIfNotSet(
     const std::vector<base::Feature>& features_to_disable) {
   auto* cl = base::CommandLine::ForCurrentProcess();
   std::vector<std::string> enabled_features;
-  for (const auto& f : base::FeatureList::SplitFeatureListString(
-           cl->GetSwitchValueASCII(switches::kEnableFeatures))) {
+  std::string enabled_features_str =
+      cl->GetSwitchValueASCII(switches::kEnableFeatures);
+  for (const auto& f :
+       base::FeatureList::SplitFeatureListString(enabled_features_str)) {
     enabled_features.emplace_back(f);
   }
 
   std::vector<std::string> disabled_features;
-  for (const auto& f : base::FeatureList::SplitFeatureListString(
-           cl->GetSwitchValueASCII(switches::kDisableFeatures))) {
+  std::string disabled_features_str =
+      cl->GetSwitchValueASCII(switches::kDisableFeatures);
+  for (const auto& f :
+       base::FeatureList::SplitFeatureListString(disabled_features_str)) {
     disabled_features.emplace_back(f);
   }