Adjust DrawingBuffer's visibility tracking behavior with lost context

Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.cpp b/Source/platform/graphics/gpu/DrawingBuffer.cpp
index ef349001ef2377de78cb5a94b580bd712aeb6ef4..f0a287601cfc6d8885faed84a7728d7c40ef6349 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -194,7 +194,7 @@ void DrawingBuffer::setIsHidden(bool hidden)
     if (m_isHidden == hidden)
         return;
     m_isHidden = hidden;
-    if (m_isHidden)
+    if (m_isHidden && !m_context->isContextLost())

[danakj, 2014/09/10 15:11:49]

Can you explain why you don't free mailboxes when the context is lost?

[Justin Novosad, 2014/09/10 17:09:50]

Had a similar issue with 2D canvas. It's because you can't delete resources on a
lost context. But it is moot anyways since lost contexts just drop everything
they own (or ref) anyways.

[danakj, 2014/09/10 17:12:12]

I'm not sure why you can't delete things? You can DeleteTexture and it "works"
just does nothing, but doesn't generate an error or anything, and lets you avoid
branches. Is that what you mean or something else?

[Justin Novosad, 2014/09/10 17:16:24]

It's the deleting of mailboxes that was causing trouble.  Perhaps that is no
longer a problem today?

I think leaving invalid mailboxes in m_recycledMailboxQueue is fine since they
will never get used anyways since the DrawingBuffer is permanently disabled once
it's context is lost.  Code might be less fragile if we cleared
m_recycledMailboxQueue (without) deleting.  So perhaps instead of doing this,
you could just add an early exit in deleteMailbox if the context is lost?  Just
a suggestion. Feel free to disregard if you disagree.

[danakj, 2014/09/10 17:18:11]

As long as we don't use them anything is okay, but I'm super curious about the
claim deleting mailboxes on a lost context causing trouble. That would be
problematic, what kind of trouble?

         freeRecycledMailboxes();
 }
 
@@ -208,10 +208,6 @@ void DrawingBuffer::freeRecycledMailboxes()
 
 bool DrawingBuffer::prepareMailbox(WebExternalTextureMailbox* outMailbox, WebExternalBitmap* bitmap)
 {
-    ASSERT(!m_isHidden);
-    if (!m_contentsChanged)
-        return false;
-
     if (m_destructionInProgress) {
         // It can be hit in the following sequence.
         // 1. WebGL draws something.
@@ -220,6 +216,9 @@ bool DrawingBuffer::prepareMailbox(WebExternalTextureMailbox* outMailbox, WebExt
         // 4. Here.
         return false;
     }
+    ASSERT(!m_isHidden);
+    if (!m_contentsChanged)
+        return false;
 
     // Resolve the multisampled buffer into m_colorBuffer texture.
     if (m_multisampleMode != None)