Simplify platform_canvas.h by recognizing that PlatformCanvas does not actually extend

skia/ext/bitmap_platform_device_linux.cc

Index: skia/ext/bitmap_platform_device_linux.cc
===================================================================
--- skia/ext/bitmap_platform_device_linux.cc	(revision 167659)
+++ skia/ext/bitmap_platform_device_linux.cc	(working copy)
@@ -176,6 +176,14 @@
   data_->SetMatrixClip(transform, region);
 }
 
+// PlatformCanvas impl
+
+SkCanvas* CreatePlatformCanvas(int width, int height, bool is_opaque,
+                               uint8_t* data, OnFailureType failureType) {
+  SkDevice* dev = BitmapPlatformDevice::Create(width, height, is_opaque, data);
+  return CreateCanvas(dev, failureType);
+}
+
 // Port of PlatformBitmap to linux
 
 PlatformBitmap::~PlatformBitmap() {