Add libx11-xcb1:i386 as a 32-bit build dep

libX11-xcb.so.1 is required to generate the v8 context shapshot.  This was
causing the LUCI version of Linux Build (dbg)(32) to fail [1].

Patch adapted from [2] from timbrown@.

[1] https://logs.chromium.org/v/?s=chromium%2Fbuildbucket%2Fcr-buildbucket.appspot.com%2F8951515267836408512%2F%2B%2Fsteps%2Fcompile%2F0%2Fstdout
[2] https://chromium-review.googlesource.com/c/chromium/src/+/940196

BUG=823962
R=dpranke
CC=timbrown

Change-Id: I8fa8f8d8281613193bc9faf2f2c0c86d900ac401
Reviewed-on: https://chromium-review.googlesource.com/972590Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544802}

build/install-build-deps.sh

@@ -365,6 +365,9 @@ fi
 # 32-bit libraries needed e.g. to compile V8 snapshot for Android or armhf
 lib32_list="linux-libc-dev:i386 libpci3:i386"
 
+# 32-bit libraries needed for a 32-bit build
+lib32_list="$lib32_list libx11-xcb1:i386"
+
 # arm cross toolchain packages needed to build chrome on armhf
 EM_REPO="deb http://emdebian.org/tools/debian/ jessie main"
 EM_SOURCE=$(cat <<EOF
@@ -569,9 +572,9 @@ else
 fi
 
 if test "$do_inst_lib32" = "1" ; then
-  echo "Including 32-bit libraries for ARM/Android."
+  echo "Including 32-bit libraries."
 else
-  echo "Skipping 32-bit libraries for ARM/Android."
+  echo "Skipping 32-bit libraries."
   lib32_list=
 fi