Fix the performance degradation after removing CString

After removing CString in blink, the performance degradation(1.6%) has been reported
on Android Nexus6 WebView Perf bot with system_health.memory_mobile benchmark.

To fix the performance regresssion, this CL makes FontFaceCreationParams::Filename()
return const std::string& to avoid a copy of std::string, And also, this CL moves
|fallback_data.filename| to |fallback_font->filename| by using std::move in
FontCache::GetFontForCharacter().

Bug: 974762, 950077
Change-Id: I7e08f56e58b96d326982e7d4d26866d4cafab881
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1701628
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678541}

third_party/blink/renderer/platform/fonts/font_face_creation_params.h

@@ -86,7 +86,7 @@ class FontFaceCreationParams {
     DCHECK_EQ(creation_type_, kCreateFontByFamily);
     return family_;
   }
-  std::string Filename() const {
+  const std::string& Filename() const {
     DCHECK_EQ(creation_type_, kCreateFontByFciIdAndTtcIndex);
     return filename_;
   }

third_party/blink/renderer/platform/fonts/linux/font_cache_linux.cc

@@ -72,7 +72,7 @@ void FontCache::GetFontForCharacter(
         gfx::GetFallbackFontForChar(c, locale);
     fallback_font->name = String::FromUTF8(fallback_data.name.data(),
                                            fallback_data.name.length());
-    fallback_font->filename = fallback_data.filename;
+    fallback_font->filename = std::move(fallback_data.filename);
     fallback_font->fontconfig_interface_id = 0;
     fallback_font->ttc_index = fallback_data.ttc_index;
     fallback_font->is_bold = fallback_data.is_bold;