[Refactor] Replace static casts to WebContentsImpl with member function.

content/DEPS

 # Do NOT add chrome to the list below. We shouldn't be including files
 # from src/chrome in src/content. Also, we may only depend on files in
 # src/components that are shared with the mojo html_viewer.
 include_rules = [
   # The subdirectories in content/ will manually allow their own include
   # directories in content/ so we disallow all of them.
   "-content",
   "+content/app/resources/grit/content_resources.h",
   "+content/common",
   "+content/grit",
@@ skipping 97 matching lines @@
 
   "+storage/browser",
   "+storage/common",
 
   # For generated JNI includes.
   "+jni",
 ]
 
 # content -> content/shell dependency is not allowed, except for browser tests.
 specific_include_rules = {
-  ".*_browsertest[a-z_]*\.cc": [
+  ".*_browsertest[a-z_]*\.(cc|h)": [
     "+content/shell/browser",
     "+content/shell/common",
   ],
 }