Mark jpeg_nbits_table as external in asm files to reduce disk impact

A recent change was made to mitigate the duplication of the
jpeg_nbits_table array in libjpeg_turbo that resulted in 64k in disk
impact for each module it was included in.  That change did not mitigate
the usage of jpeg_nbits_table in the assembly files in that library,
thus the duplication remains.  This change removes the definition of the
array in those files and replaces it with a declaration of an external
reference.

Previous change: http://crrev.com/c/1685791

Bug: http://crbug.com/992072
Change-Id: I6c7f1b3a665f36b81eacd32050f69b4dca932b1e
diff --git a/README.chromium b/README.chromium
index 34b867e..21db863 100644
--- a/README.chromium
+++ b/README.chromium
@@ -46,7 +46,9 @@
   was in the jchuff.obj and jcphuff.obj resulting in an added 65k in
   .rdata in chrome.dll and chrome_child.dll.  Declaring extern const
   in the header instead of static const and moving the definition to
-  a new .c file fixes this so only one copy is referenced.
+  a new .c file fixes this so only one copy is referenced. Also added
+  extern wrappers around usage in asm files. The jpeg_nbits_table.inc
+  file was also deleted.
 * Arm NEON patches to improve performance and maintainability. These changes
   are tracked by the following Chromium issue: https://crbug.com/922430
   - Add memory alignment size check in jmemmgr.c