When casting input for loads, include modifiers such as 'const'

Clears build warnings:

warning: cast from type 'const uint8_t* {aka const unsigned char*}' to
type 'uint32_t* {aka unsigned int*}' casts away qualifiers [-Wcast-qual]

Bug: webm:1509, libyuv:791
Test: g++ -Iinclude -I../libvpx/third_party/libwebm -I../libvpx/vp8 -I../libvpx/vp8 -I../libvpx/vp9 -I../libvpx/vp9 -Iinclude -m64 -DNDEBUG -O3 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -Wall -Wdisabled-optimization -Wfloat-conversion -Wpointer-arith -Wtype-limits -Wcast-qual -Wvla -Wuninitialized -Wunused -Wextra -I. -I"../libvpx" -D__STDC_CONSTANT_MACROS -D__STDC_LIMIT_MACROS -Wno-unused-parameter -c -o third_party/libyuv/source/row_common.cc.o source/row_common.cc




Change-Id: I1e3b2fe2a4ae9dd466c3db9cde0560aceb9d1398
Reviewed-on: https://chromium-review.googlesource.com/996393Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>