Set proper PPAPI SandBoxSupport thread check

Replace current ThreadId check with a sequence checker.

We also add a test that tests font fallback, so
PpapiBlinkPlatformImpl::SandboxSupport::GetFallbackFontForCharacter
actually gets tested somewhere.

Bug: 866645
Change-Id: I743eb9cd6a1004c25074b907a50e9b4cf680f805
Reviewed-on: https://chromium-review.googlesource.com/1165312
Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581302}

content/ppapi_plugin/ppapi_blink_platform_impl.cc

@@ -66,9 +66,9 @@ class PpapiBlinkPlatformImpl::SandboxSupport : public WebSandboxSupport {
   // unicode code points. It needs this information frequently so we cache it
   // here.
   std::map<int32_t, blink::WebFallbackFont> unicode_font_families_;
-  // For debugging crbug.com/312965
-  base::PlatformThreadId creation_thread_;
   sk_sp<font_service::FontLoader> font_loader_;
+  // For debugging https://crbug.com/312965
+  base::SequenceCheckerImpl creation_thread_sequence_checker_;
 #endif
 };
 
@@ -86,17 +86,15 @@ bool PpapiBlinkPlatformImpl::SandboxSupport::LoadFont(CTFontRef src_font,
 
 #elif defined(OS_POSIX)
 
-PpapiBlinkPlatformImpl::SandboxSupport::SandboxSupport()
-    : creation_thread_(base::PlatformThread::CurrentId()) {
-}
+PpapiBlinkPlatformImpl::SandboxSupport::SandboxSupport() {}
 
 void PpapiBlinkPlatformImpl::SandboxSupport::GetFallbackFontForCharacter(
     WebUChar32 character,
     const char* preferred_locale,
     blink::WebFallbackFont* fallbackFont) {
   ppapi::ProxyLock::AssertAcquired();
   // For debugging crbug.com/312965
-  CHECK_EQ(creation_thread_, base::PlatformThread::CurrentId());
+  CHECK(creation_thread_sequence_checker_.CalledOnValidSequence());
   const std::map<int32_t, blink::WebFallbackFont>::const_iterator iter =
       unicode_font_families_.find(character);
   if (iter != unicode_font_families_.end()) {

ppapi/tests/test_browser_font.cc

@@ -54,8 +54,12 @@ std::string TestBrowserFont::TestMeasure() {
   int32_t length1 = font.MeasureText(pp::BrowserFontTextRun("WWW"));
   ASSERT_TRUE(length1 > 0);
   int32_t length2 = font.MeasureText(pp::BrowserFontTextRun("WWWWWWWW"));
-
   ASSERT_TRUE(length2 >= length1 * 2);
+
+  // Fallback font test.
+  int32_t length3 = font.MeasureText(pp::BrowserFontTextRun("こんにちは"));
+  ASSERT_TRUE(length3 > 0);
+
   PASS();
 }