Add libjpeg_turbo dir override for ANGLE

ANGLE has provided angle_libjpeg_turbo_dir as a build override for a
long period, but that probably never had any real effect until
crrev.com/c/2282277.

Bug: None
Change-Id: I44228ed1403cc496beeb00410a337f4f7ce538dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2285839Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Jiajie Hu <jiajie.hu@intel.com>
Cr-Commit-Position: refs/heads/master@{#786598}

build_overrides/angle.gni

@@ -12,6 +12,7 @@ angle_has_build = true
 angle_glslang_dir = "//third_party/glslang/src"
 angle_googletest_dir = "//third_party/googletest/src"
 angle_jsoncpp_dir = "//third_party/jsoncpp"
+angle_libjpeg_turbo_dir = "//third_party/libjpeg_turbo"
 angle_libpng_dir = "//third_party/libpng"
 angle_spirv_cross_dir = "//third_party/spirv-cross/spirv-cross"
 angle_spirv_headers_dir = "//third_party/spirv-headers/src"