Use a static variable to store the Chrome Product Version

The Chrome Product Version that is returned by
version_info::GetProductNameAndVersionForUserAgent is a static string
that can never change during the course of a running binary, since the
PRODUCT_VERSION it is based off of is a macro generated at build time.

Instead of recreating this string on every invocation, we store it in a
static constant.

This CL also cleans up some unnecessary namespace qualifications.

Change-Id: Ida7758f1e58c77778d4b2c0c664c85005b16a41b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3169075
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Ali Beyad <abeyad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#923404}

components/version_info/version_info.cc

@@ -17,8 +17,10 @@
 
 namespace version_info {
 
-std::string GetProductNameAndVersionForUserAgent() {
-  return "Chrome/" + GetVersionNumber();
+const std::string& GetProductNameAndVersionForUserAgent() {
+  static const base::NoDestructor<std::string> product_and_version(
+      "Chrome/" + GetVersionNumber());
+  return *product_and_version;
 }
 
 std::string GetProductName() {

components/version_info/version_info.h

@@ -17,7 +17,7 @@ namespace version_info {
 
 // Returns the product name and version information for UserAgent header,
 // e.g. "Chrome/a.b.c.d".
-std::string GetProductNameAndVersionForUserAgent();
+const std::string& GetProductNameAndVersionForUserAgent();
 
 // Returns the product name, e.g. "Chromium" or "Google Chrome".
 std::string GetProductName();