Remove never-reached if block in StyleSheetContents::WrapperDeleteRule

|StyleSheetContents::import_rules_| is of type
HeapVector<Member<StyleRuleImport>>. Its members will always
resolve |import_rules_[index]->IsFontFaceRule()| to false (once
StyleRuleBase::IsImportRule() is the one that resolves to true.

This CL removes the if-block that checks that altogether,
allowing the removal of the remaining DEFINE_STYLE_RULE_TYPE_CASTS
occurrence.

Note: This was caught because trying to use To<StyleRuleFaceRule>
failed to compile.

  ../../third_party/blink/renderer/platform/wtf/casting.h:102:10: error:
  non-const lvalue reference to type 'blink::StyleRuleFontFace'
  cannot bind to a value of unrelated type 'blink::StyleRuleImport'
  return static_cast<Derived&>(from);
         ^                     ~~~~

R=jbroman@chromium.org
CC=​blink-reviews-vendor@chromium.org

BUG=891908

Change-Id: I6bbf992b3654601a50fc68b18961de79899d5284
Reviewed-on: https://chromium-review.googlesource.com/c/1496056
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#636879}
2 files changed