[Cleanup] Un-const the return of various Extension methods

Extension::VersionString(), Extension::DifferentialFingerprint(), and
Extension::GetVersionForDisplay() returned a `const std::string`;
there's no reason to do this, since it's a new string (and not a
reference to a member). Additionally, this could potentially result in
invocations of these methods creating a copy, if the caller did
something like:

std::string version_string = extension.VersionString();

Since the result would be a `const std::string`, a copy might be made to
fit into the non-const std::string required by the caller. Hopefully,
the compiler is smart enough to optimize this out, but it's better to
just un-const the result altogether.

Bug: None
Change-Id: Ic586e8e7f907431ddf5ee3efb7b111b9704c4144
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919731Reviewed-by: Kelvin Jiang <kelvinjiang@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716895}

extensions/common/extension.cc

@@ -417,11 +417,11 @@ const HashedExtensionId& Extension::hashed_id() const {
   return manifest_->hashed_id();
 }
 
-const std::string Extension::VersionString() const {
+std::string Extension::VersionString() const {
   return version_.GetString();
 }
 
-const std::string Extension::DifferentialFingerprint() const {
+std::string Extension::DifferentialFingerprint() const {
   std::string fingerprint;
   // We currently support two sources of differential fingerprints:
   // server-provided and synthesized. Fingerprints are of the format V.FP, where
@@ -435,7 +435,7 @@ const std::string Extension::DifferentialFingerprint() const {
              : "2." + VersionString();
 }
 
-const std::string Extension::GetVersionForDisplay() const {
+std::string Extension::GetVersionForDisplay() const {
   if (version_name_.size() > 0)
     return version_name_;
   return VersionString();

extensions/common/extension.h

@@ -260,9 +260,9 @@ class Extension : public base::RefCountedThreadSafe<Extension> {
   const HashedExtensionId& hashed_id() const;
   const base::Version& version() const { return version_; }
   const std::string& version_name() const { return version_name_; }
-  const std::string VersionString() const;
-  const std::string DifferentialFingerprint() const;
-  const std::string GetVersionForDisplay() const;
+  std::string VersionString() const;
+  std::string DifferentialFingerprint() const;
+  std::string GetVersionForDisplay() const;
   const std::string& name() const { return display_name_; }
   const std::string& short_name() const { return short_name_; }
   const std::string& non_localized_name() const { return non_localized_name_; }