fix WBMP image will get null color table when draw in RGB565 Canvas

Symptom: draw Index8 to RGB565 will using SkColorTable::lock16BitCache and return a null array.
Root Cause:lock16BitCache return null pointer because default type is kPremul_SkAlphaType
Solution: WBMP color table should be kOpaque_SkAlphaType
Reproduce steps: draw a WBMP with a matrix that apply rotate and transform

This patch was proposed by arthur_hung at htc.com to the AOSP project.

R=reed@google.com, scroggo@google.com

Review URL: https://codereview.chromium.org/377443007

src/images/SkImageDecoder_wbmp.cpp

diff --git a/src/images/SkImageDecoder_wbmp.cpp b/src/images/SkImageDecoder_wbmp.cpp
index 0bf1389..ddb7c3d 100644
--- a/src/images/SkImageDecoder_wbmp.cpp
+++ b/src/images/SkImageDecoder_wbmp.cpp
@@ -119,7 +119,7 @@
     }
 
     const SkPMColor colors[] = { SK_ColorBLACK, SK_ColorWHITE };
-    SkColorTable* ct = SkNEW_ARGS(SkColorTable, (colors, 2));
+    SkColorTable* ct = SkNEW_ARGS(SkColorTable, (colors, 2, kOpaque_SkAlphaType));
     SkAutoUnref   aur(ct);
 
     if (!this->allocPixelRef(decodedBitmap, ct)) {