Use PDFium V8 flag recommendations in chromium.

Avoids making changes in both chromium and pdfium repositories
as these requirements evolve.

Change-Id: I83dc0c4e272d6c2be23c290b9838ebeef37269ca
Reviewed-on: https://chromium-review.googlesource.com/c/1355942
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612723}

pdf/pdfium/pdfium_engine.cc

@@ -533,8 +533,8 @@ std::string GetDocumentMetadata(FPDF_DOCUMENT doc, const std::string& key) {
 gin::IsolateHolder* g_isolate_holder = nullptr;
 
 void SetUpV8() {
-  static const char kNoExposeWasm[] = "--no-expose-wasm";
-  v8::V8::SetFlagsFromString(kNoExposeWasm, strlen(kNoExposeWasm));
+  const char* recommended = FPDF_GetRecommendedV8Flags();
+  v8::V8::SetFlagsFromString(recommended, strlen(recommended));
 
   gin::IsolateHolder::Initialize(gin::IsolateHolder::kNonStrictMode,
                                  gin::IsolateHolder::kStableV8Extras,