[LSC] Replace base::string16 with std::u16string in //[a-e]*

This change replaces base::string16 with std::u16string in //[a-e]*.

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

Bug: 1184339
Change-Id: I7930c2c3ae510b6d2232cc1b3a927ad90515ffe9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2752407
Commit-Queue: Jan Wilken Dörrie <[email protected]>
Owners-Override: Jan Wilken Dörrie <[email protected]>
Reviewed-by: Peter Kasting <[email protected]>
Cr-Commit-Position: refs/heads/master@{#862191}
diff --git a/docs/ios/build_instructions.md b/docs/ios/build_instructions.md
index 3440d30..09e356b 100644
--- a/docs/ios/build_instructions.md
+++ b/docs/ios/build_instructions.md
@@ -350,7 +350,7 @@
 ```
 
 This will also allow you to see the content of some of Chromium types in the
-debugger like `base::string16`, ... If you want to use `lldb` directly, name
+debugger like `std::u16string`, ... If you want to use `lldb` directly, name
 the file `~/.lldbinit` instead of `~/.lldbinit-Xcode`.
 
 Note: if you are using `ios/build/tools/setup-gn.py` to generate the Xcode