Add RAW decoding into Skia.

TBR=reed@google.com

BUG=skia:

(Based on the work from ebrauer in https://codereview.chromium.org/1459473007)
(Based on the work from adaubert in https://codereview.chromium.org/1494003003)
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1520403003

Committed: https://skia.googlesource.com/skia/+/6bd8639f8c142eedf543f4e5f3b02d2bf11df308

Review URL: https://codereview.chromium.org/1520403003
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index 4cab958..abbe729 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -37,6 +37,11 @@
   #
   'variables': {  # level 1
     'angle_path%': '../',
+
+    # RAW codec needs exceptions. Due to that, it is a separate target. Its usage can be controlled
+    # by this variable.
+    'skia_codec_decodes_raw%': 1,
+
     'variables': {  # level 2
 
       # Variables needed by conditions list within the level-2 variables dict.