libpng: optimize for speed on Arm platforms

Change libpng build configuration to optimize for speed on Arm
platforms. This allows the compiler to inline some of the NEON filter
functions and significantly increase performance.

Binary size increase is minimal (<10KB increase in normalized Android
APK size.)

Bug: 1071398
Change-Id: I109dea8b490bfdd2c0a80e2f1259797f1485e14f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2149331Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Chris Blume <cblume@chromium.org>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
Cr-Commit-Position: refs/heads/master@{#760551}

third_party/libpng/BUILD.gn

@@ -82,6 +82,8 @@ source_set("libpng_sources") {
       "PNG_ARM_NEON_OPT=2",
       "PNG_ARM_NEON_IMPLEMENTATION=1",
     ]
+    configs -= [ "//build/config/compiler:default_optimization" ]
+    configs += [ "//build/config/compiler:optimize_speed" ]
   } else if (current_cpu == "mips64el" && mips_use_msa) {
     sources += [
       "mips/filter_msa_intrinsics.c",