ignore SK_DISABLE_READBUFFER

Its effect is small and we want to rely on SkReadBuffer being available.

Size changes:

    - canvaskit uncompressed: 6,864,481 --> 6,864,481  (  no change  )
    - canvaskit   compressed: 2,667,117 --> 2,667,117  (  no change  )
    -   pathkit uncompressed:   329,187 -->   330,679  (+ 1.5K, +0.5%)
    -   pathkit   compressed:   134,158 -->   134,672  (+ 0.5K, +0.4%)
    -   flutter             : 1,302,108 --> 1,322,568  (+20.0K, +1.6%)

The Flutter change is the biggest mystery, as bloaty only pegs
SkReadBuffer as 3.9K.  The rest must come from other files including
SkReadBuffer.h not being able to see and inline away SkReadBuffer
routines?  Feels like SK_DISABLE_EFFECT_DESERIALIZATION isn't trimming
enough?

PS 4-6 have an idea to push SK_DISABLE_EFFECT_DESERIALIZATION further.

Change-Id: Ifda3ccb82dd0636cfed6bb826fb185a7bca2cbe0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295061
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 7f9c796..381969f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1152,6 +1152,7 @@
     "src/core/SkPathRef.cpp",
     "src/core/SkPoint.cpp",
     "src/core/SkRRect.cpp",
+    "src/core/SkReadBuffer.cpp",
     "src/core/SkRect.cpp",
     "src/core/SkSemaphore.cpp",
     "src/core/SkStream.cpp",