Bug 776505. Assert that nsDeviceContext is created on the main thread only. r=mats

--HG--
extra : rebase_source : fcda449b3bf25c9827c2fa2f000c20460819ce53

gfx/src/nsDeviceContext.cpp

@@ -226,6 +226,7 @@ nsDeviceContext::nsDeviceContext()
       mPixelScale(1.0f), mPrintingScale(1.0f),
       mFontCache(nullptr)
 {
+    MOZ_ASSERT(NS_IsMainThread(), "nsDeviceContext created off main thread");
 }
 
 // Note: we use a bare pointer for mFontCache so that nsFontCache