Revert "Update libwebp to 0.6.0-pre"

This reverts commit e2f6ffbf49671713356881af70a093e99e80bd31.

Reason for revert:

iOS build failures, see https://luci-milo.appspot.com/swarming/task/33e5e02cf8419d10/steps/build_iOSShell/0/stdout

Original change's description:
> Update libwebp to 0.6.0-pre
> 
> Corresponds with Android change
> https://android-review.googlesource.com/#/c/326439/
> 
> "this prerelease snapshot includes encoder and performance
> improvements"
> 
> Update build file
> * Many files have been renamed from
>     src/<subdir>/<name>.c
>       to
>     src/<subdir>/<name>_<subdir>.c
> 
> * Build new files (*_msa.c, *_neon.c, predictor_enc.c)
> 
> This should fix issue 5876, which was caused by a compiler bug.
> With the added NEON implementation, we will no longer trigger
> the bug.
> 
> BUG=skia:5876
> 
> Change-Id: I0fcce4362ee70138547b1d23aa9ef537a4126e73
> Reviewed-on: https://skia-review.googlesource.com/7376
> Reviewed-by: Matt Sarett <msarett@google.com>
> Commit-Queue: Leon Scroggins <scroggo@google.com>
> 

TBR=msarett@google.com,scroggo@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:5876

Change-Id: Ib81f63861d9f1c6936bd7f790b5e16a9544a4df0
Reviewed-on: https://skia-review.googlesource.com/7420
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
diff --git a/DEPS b/DEPS
index b89bc0f..f42e7d4 100644
--- a/DEPS
+++ b/DEPS
@@ -15,7 +15,7 @@
   "third_party/externals/gyp"     : "https://chromium.googlesource.com/external/gyp.git@87ac4d0e63fc7dd8152a350327fea8dcf031bf56",
   "third_party/externals/harfbuzz": "https://skia.googlesource.com/third_party/harfbuzz.git@1.3.0",
   "third_party/externals/jsoncpp" : "https://chromium.googlesource.com/external/github.com/open-source-parsers/jsoncpp.git@1.0.0",
-  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@84cef16",
+  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@v0.5.2-rc2",
   "third_party/externals/zlib"    : "https://chromium.googlesource.com/chromium/src/third_party/zlib@4576304a4b9835aa8646c9735b079e1d96858633",
 
   "third_party/externals/dng_sdk" : "https://android.googlesource.com/platform/external/dng_sdk.git@96443b262250c390b0caefbf3eed8463ba35ecae",