Allocate TextureReadLock at TextureClient creation and drop file handles immediately after. (bug 1416726, r=aosmond)

This changes the lifecycle and API for TextureReadLock to fix file descriptor exhaustion
crashes. These changes are partially superficial and mostly align the API of TextureReadLocks
with their actual usage.

The changes are:

1. Create the TextureReadLock in the TextureClient constructor so it's available before IPC creation
    a. This is superficial as EnableReadLock was always called before IPC creation
2. Send the ReadLockDescriptor in the PTextureConstructor message and close the file handle
3. Receive the ReadLockDescriptor in TextureHost and close the file handle
4. Send a boolean flag in layer transactions if the texture is read locked instead of a descriptor
5. Use a boolean flag in TextureHost to determine if the ReadLock must be unlocked instead of a nullptr

I believe that we can remove the InitReadLocks code from LayerTransaction as that was added to
prevent file descriptor limits in IPDL messages and is no longer needed with this change. But
that is a non-essential change and this patch is already big enough.

MozReview-Commit-ID: DzHujrOQejH

gfx/layers/client/ContentClient.cpp

@@ -750,14 +750,13 @@ ContentClientRemoteBuffer::CreateBufferInternal(const gfx::IntRect& aRect,
 
   RefPtr<TextureClient> textureClient = CreateTextureClientForDrawing(
     aFormat, aRect.Size(), BackendSelector::Content,
-    aFlags | ExtraTextureFlags(),
+    aFlags | ExtraTextureFlags() | TextureFlags::BLOCKING_READ_LOCK,
     textureAllocFlags
   );
 
   if (!textureClient || !AddTextureClient(textureClient)) {
     return nullptr;
   }
-  textureClient->EnableBlockingReadLock();
 
   RefPtr<TextureClient> textureClientOnWhite;
   if (aFlags & TextureFlags::COMPONENT_ALPHA) {