Revert "Remove all KTX support"

This reverts commit ada12ab0557a1f540e775288e2ce44f64fcd8c7e.

Reason for revert: Google3 needs it:
https://test.corp.google.com/ui#id=OCL:142184832:BASE:142184975:1481839118985:32fde8ef

Original change's description:
> Remove all KTX support
> 
> It is untested and unused.
> 
> Change-Id: I010ff4ad942738f362d42a99af4edbbb1cb0cd71
> Reviewed-on: https://skia-review.googlesource.com/6142
> Commit-Queue: Leon Scroggins <scroggo@google.com>
> Reviewed-by: Mike Klein <mtklein@chromium.org>
> Reviewed-by: Robert Phillips <robertphillips@google.com>
> 

TBR=mtklein@chromium.org,mtklein@google.com,robertphillips@google.com,scroggo@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I1ea2f9487eb2212efbfcc514122792b70c9e8737
Reviewed-on: https://skia-review.googlesource.com/6181
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 67fe9f5..a048092 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -110,6 +110,7 @@
     "src/utils/win",
     "third_party/etc1",
     "third_party/gif",
+    "third_party/ktx",
   ]
 
   defines = [
@@ -589,6 +590,7 @@
     "src/utils/mac/SkStream_mac.cpp",
     "third_party/etc1/etc1.cpp",
     "third_party/gif/SkGifImageReader.cpp",
+    "third_party/ktx/ktx.cpp",
   ]
 
   libs = []