[ios] Stop using global static std::string objects

Using global static non-POD object is against the style guide when
they have a destructor (which std::string does) as the destructors
are called in an order that is not controllable and they can cause
issue during shutdown.

Bug: none
Change-Id: I8967d86d029004bc66912ae6b28f2be59633dd08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2640535
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: David Jean <djean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846067}

ios/chrome/browser/sessions/session_restoration_browser_agent.mm

@@ -36,7 +36,8 @@
 BROWSER_USER_DATA_KEY_IMPL(SessionRestorationBrowserAgent)
 
 namespace {
-const std::string kSessionDirectory = "Sessions";
+const base::FilePath::CharType kSessionDirectory[] =
+    FILE_PATH_LITERAL("Sessions");
 }
 
 // static

ios/chrome/browser/snapshots/snapshot_browser_agent.mm

@@ -20,9 +20,12 @@
 
 namespace {
 
-const std::string kLegacyBaseDirectory = "Chromium";
-const std::string kSessionsDirectory = "Sessions";
-const std::string kSnapshotsDirectory = "Snapshots";
+const base::FilePath::CharType kLegacyBaseDirectory[] =
+    FILE_PATH_LITERAL("Chromium");
+const base::FilePath::CharType kSessionsDirectory[] =
+    FILE_PATH_LITERAL("Sessions");
+const base::FilePath::CharType kSnapshotsDirectory[] =
+    FILE_PATH_LITERAL("Snapshots");
 
 }  // namespace