[LSC] Replace base::string16 with std::u16string in //{chrome*,content}

This change replaces base::string16 with std::u16string in //chrome,
//chromecast, //chromeos and //content.

Reproduction steps:
$ git grep -lw 'base::string16' chrome* content | \
      xargs sed -i 's/\bbase::string16\b/std::u16string/g'
$ git cl format

Bug: 1184339
Change-Id: I8fae0ab25b5d9bf1cb416ae5f47d7f680fb8f3ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2752229
Commit-Queue: Jan Wilken Dörrie <[email protected]>
Commit-Queue: Daniel Cheng <[email protected]>
Owners-Override: Daniel Cheng <[email protected]>
Auto-Submit: Jan Wilken Dörrie <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Cr-Commit-Position: refs/heads/master@{#862152}
diff --git a/content/shell/browser/shell_platform_delegate_aura.cc b/content/shell/browser/shell_platform_delegate_aura.cc
index 61006b9..13ee15e 100644
--- a/content/shell/browser/shell_platform_delegate_aura.cc
+++ b/content/shell/browser/shell_platform_delegate_aura.cc
@@ -80,7 +80,7 @@
 void ShellPlatformDelegate::SetIsLoading(Shell* shell, bool loading) {}
 
 void ShellPlatformDelegate::SetTitle(Shell* shell,
-                                     const base::string16& title) {}
+                                     const std::u16string& title) {}
 
 void ShellPlatformDelegate::MainFrameCreated(Shell* shell) {}