skia: add ability to load command_buffer_gles2

BUG=skia:

Review URL: https://codereview.chromium.org/1306823003
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 44daeb6..5205007 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -81,7 +81,9 @@
 SkTCPServer gServer;
 #endif
 
-#if SK_ANGLE
+#if SK_COMMAND_BUFFER
+#define DEFAULT_TO_COMMAND_BUFFER 1
+#elif SK_ANGLE
 //#define DEFAULT_TO_ANGLE 1
 #else
 #define DEFAULT_TO_GPU 0 // if 1 default rendering is on GPU
@@ -197,6 +199,12 @@
                 fBackend = kANGLE_BackEndType;
                 break;
 #endif // SK_ANGLE
+#if SK_COMMAND_BUFFER
+            case kCommandBuffer_DeviceType:
+                // Command buffer is really the only other odd man out :D
+                fBackend = kCommandBuffer_BackEndType;
+                break;
+#endif // SK_COMMAND_BUFFER
             default:
                 SkASSERT(false);
                 break;
@@ -223,6 +231,11 @@
                 glInterface.reset(GrGLCreateANGLEInterface());
                 break;
 #endif // SK_ANGLE
+#if SK_COMMAND_BUFFER
+            case kCommandBuffer_DeviceType:
+                glInterface.reset(GrGLCreateCommandBufferInterface());
+                break;
+#endif // SK_COMMAND_BUFFER
             default:
                 SkASSERT(false);
                 break;
@@ -670,6 +683,9 @@
 #if SK_ANGLE
         SampleWindow::kANGLE_DeviceType,
 #endif // SK_ANGLE
+#if SK_COMMAND_BUFFER
+        SampleWindow::kCommandBuffer_DeviceType,
+#endif // SK_COMMAND_BUFFER
 #endif // SK_SUPPORT_GPU
         SampleWindow::kRaster_DeviceType,
     };
@@ -790,6 +806,9 @@
 #if SK_ANGLE && DEFAULT_TO_ANGLE
     fDeviceType = kANGLE_DeviceType;
 #endif
+#if SK_COMMAND_BUFFER && DEFAULT_TO_COMMAND_BUFFER
+    fDeviceType = kCommandBuffer_DeviceType;
+#endif
 
     fUseClip = false;
     fAnimating = false;
@@ -830,6 +849,9 @@
 #if SK_ANGLE
                                 "ANGLE",
 #endif
+#if SK_COMMAND_BUFFER
+                                "Command Buffer",
+#endif
                                 nullptr);
     fAppMenu->assignKeyEquivalentToItem(itemID, 'd');
     itemID = fAppMenu->appendTriState("AA", "AA", sinkID, fAAState);
@@ -1939,6 +1961,9 @@
 #if SK_ANGLE
     "angle: ",
 #endif // SK_ANGLE
+#if SK_COMMAND_BUFFER
+    "command buffer: ",
+#endif // SK_COMMAND_BUFFER
 #endif // SK_SUPPORT_GPU
 };
 static_assert(SK_ARRAY_COUNT(gDeviceTypePrefix) == SampleWindow::kDeviceTypeCnt,