Begin atlasing

This CL makes it possible for pulled-forward-layers to be atlased. It currently has a couple glaring limitations (which is why it is disabled):

1) the atlased layers cannot be purged nor aged out
2) the texture backing the atlas is not pulled from (or returned to) the resource cache

#1 is on hold until we have a recycling rectanizer

A separate major limitation (the non-atlased layers aren't cached) is blocked until we can transmute entries in the resource cache from scratch to non-scratch while potentially preserving their contents.

R=bsalomon@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/354533004
diff --git a/src/gpu/GrTextStrike.cpp b/src/gpu/GrTextStrike.cpp
index fb9d631..a9405ca 100644
--- a/src/gpu/GrTextStrike.cpp
+++ b/src/gpu/GrTextStrike.cpp
@@ -82,7 +82,7 @@
     if (NULL == fAtlases[atlasIndex]) {
         SkISize textureSize = SkISize::Make(GR_ATLAS_TEXTURE_WIDTH,
                                             GR_ATLAS_TEXTURE_HEIGHT);
-        fAtlases[atlasIndex] = SkNEW_ARGS(GrAtlas, (fGpu, config,
+        fAtlases[atlasIndex] = SkNEW_ARGS(GrAtlas, (fGpu, config, kNone_GrTextureFlags,
                                                     textureSize,
                                                     GR_NUM_PLOTS_X,
                                                     GR_NUM_PLOTS_Y,