Merge to XFA: Allow compiling PDFium without V8.

Original Review URL: https://codereview.chromium.org/1395733006 .
(cherry picked from commit f1c713663192368d26031a4caed1f9705f4510af)

Conflicts:
  BUILD.gn
  fpdfsdk/src/fpdfview.cpp
  fpdfsdk/src/javascript/JS_Runtime.cpp
  pdfium.gyp
  samples/BUILD.gn
  samples/samples.gyp

BUG=pdfium:211
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1393833006 .
diff --git a/fpdfsdk/include/javascript/IJavaScript.h b/fpdfsdk/include/javascript/IJavaScript.h
index fc8bf2b..f78ee52 100644
--- a/fpdfsdk/include/javascript/IJavaScript.h
+++ b/fpdfsdk/include/javascript/IJavaScript.h
@@ -134,6 +134,7 @@
 
 class IJS_Runtime {
  public:
+  static void Initialize(unsigned int slot, void* isolate);
   static IJS_Runtime* Create(CPDFDoc_Environment* pEnv);
   virtual ~IJS_Runtime() {}