[LSC] Replace base::string16 with std::u16string in //[f-z]*

This change replaces base::string16 with std::u16string in //[f-z]*

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

Bug: 1184339
Change-Id: I4e0ff2fd300c9fd9360dbf8f4b44e74f5f511797
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2752148
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Owners-Override: Jan Wilken Dörrie <jdoerrie@chromium.org>
Owners-Override: danakj <danakj@chromium.org>
Auto-Submit: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#862044}

diff --git a/ipc/ipc_message_unittest.cc b/ipc/ipc_message_unittest.cc
index 63503a3c..33fa3e3 100644
--- a/ipc/ipc_message_unittest.cc
+++ b/ipc/ipc_message_unittest.cc

@@ -37,7 +37,7 @@
 TEST(IPCMessageTest, BasicMessageTest) {
   int v1 = 10;
   std::string v2("foobar");
-  base::string16 v3(base::ASCIIToUTF16("hello world"));
+  std::u16string v3(base::ASCIIToUTF16("hello world"));
 
   IPC::Message m(0, 1, IPC::Message::PRIORITY_NORMAL);
   m.WriteInt(v1);
@@ -48,7 +48,7 @@
 
   int vi;
   std::string vs;
-  base::string16 vs16;
+  std::u16string vs16;
 
   EXPECT_TRUE(iter.ReadInt(&vi));
   EXPECT_EQ(v1, vi);