Fix potential use-after-free in CSSFontFaceSource

This fixes a bug introduced by crrev.com/c/824172.

Bug: 778352
Change-Id: Id1dbc552ed5a94a0b5fadddec0b5bea50c3a7597
Reviewed-on: https://chromium-review.googlesource.com/826662Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524007}

third_party/WebKit/Source/core/css/CSSFontFaceSource.cpp

@@ -78,8 +78,8 @@ void CSSFontFaceSource::PruneOldestIfNeeded() {
   if (font_cache_key_age.size() > kMaxCachedFontData) {
     DCHECK_EQ(font_cache_key_age.size() - 1, kMaxCachedFontData);
     FontCacheKey& key = font_cache_key_age.back();
-    font_cache_key_age.pop_back();
     auto font_data_entry = font_data_table_.Take(key);
+    font_cache_key_age.pop_back();
     DCHECK_EQ(font_cache_key_age.size(), kMaxCachedFontData);
     if (font_data_entry && font_data_entry->GetCustomFontData())
       font_data_entry->GetCustomFontData()->ClearFontFaceSource();