GURL::Replacements methods accept a StringPiece instead of std::string&.

Previously, it was unsafe to pass a char* because it would be implicitly
converted to a string with a very short lifetime. Now, you can safely
pass a char* as long as the memory pointed to by the char* outlives the
Replacements (which is always true for string literals).

All existing usage of Replacements should continue to work as usual.

Updated every call to Replacements Set*Str methods that that could be
simplified by the new StringPiece API (for example, passing string
literals directly, instead of copying them into a string object).

Internally, renamed StdStringReplacements to StringPieceReplacements.

BUG=454274

Review URL: https://codereview.chromium.org/889463003

Cr-Commit-Position: refs/heads/master@{#314771}
diff --git a/chrome/browser/extensions/app_process_apitest.cc b/chrome/browser/extensions/app_process_apitest.cc
index a189991..09a0e0c 100644
--- a/chrome/browser/extensions/app_process_apitest.cc
+++ b/chrome/browser/extensions/app_process_apitest.cc
@@ -49,8 +49,7 @@
   // as in the test apps manifests.
   GURL GetTestBaseURL(const std::string& test_directory) {
     GURL::Replacements replace_host;
-    std::string host_str("localhost");  // must stay in scope with replace_host
-    replace_host.SetHostStr(host_str);
+    replace_host.SetHostStr("localhost");
     GURL base_url = embedded_test_server()->GetURL(
         "/extensions/api_test/" + test_directory + "/");
     return base_url.ReplaceComponents(replace_host);