Skip returning fallback font if SkFontMgr unavailable

Work around a crash reported in issue 1022968 occurring mostly on Windows
7 SP1 systems where SkFontMgr is unable to get instantiated. If that
happens, do not attempt calling matchFamilyStyleCharacter() on a null
SkFontMgr.

Bug: 1022968
Change-Id: I5358fe982ba85c65285b965a816a16f94aa11287
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1911783Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714461}

content/browser/renderer_host/dwrite_font_proxy_impl_win.cc

@@ -517,12 +517,15 @@ void DWriteFontProxyImpl::FallbackFamilyAndStyleForCodepoint(
                                                 /* width */ 0,
                                                 /* slant */ 0));
 
-  if (!codepoint)
+  if (!codepoint || !collection_ || !factory_)
     return;
 
   sk_sp<SkFontMgr> font_mgr(
       SkFontMgr_New_DirectWrite(factory_.Get(), collection_.Get()));
 
+  if (!font_mgr)
+    return;
+
   const char* bcp47_locales[] = {locale_name.c_str()};
   int num_locales = locale_name.empty() ? 0 : 1;
   const char** locales = locale_name.empty() ? nullptr : bcp47_locales;