make allocPixels throw on failure

BUG=skia:
R=mtklein@google.com, fmalita@google.com, fmalita@chromium.org

Author: reed@google.com

Review URL: https://codereview.chromium.org/510423005
diff --git a/tests/KtxTest.cpp b/tests/KtxTest.cpp
index bddd09a..32dacd5 100644
--- a/tests/KtxTest.cpp
+++ b/tests/KtxTest.cpp
@@ -28,8 +28,7 @@
     SkRandom rand(0x1005cbad);
 
     SkBitmap bm8888;
-    bool pixelsAllocated = bm8888.allocN32Pixels(128, 128);
-    REPORTER_ASSERT(reporter, pixelsAllocated);
+    bm8888.allocN32Pixels(128, 128);
 
     uint8_t *pixels = reinterpret_cast<uint8_t*>(bm8888.getPixels());
     REPORTER_ASSERT(reporter, NULL != pixels);