Streamline SIMD targets in media.gyp

Prevents duplicates of the media_sse and yuv_convert targets from
ending up in both media and media_unittests during shared builds.
- Removes the yuv_convert target since everyone who uses it already
uses media.
- Merges differ_block_sse2 and yuv_convert_simd_x86 into media_sse2.
- Moves assembly into media_asm.
- Moves incorrect mmx bundling from sse2 to new media_mmx target.
- Introduces EXPORT macro to x86inc.asm
- Introduces yasm_includes for non-.asm files in yasm_compile.gypi.
- Fixes missing const on yuv constants table.

BUG=none
TEST=compiles, all unittests pass.
TBR=fischman@chromium.org, kbr@chromium.org, sergeyu@chromium.org

Review URL: https://codereview.chromium.org/15151002

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 14b45ec2315b8f30607b6583b5dd762958ca7ebf
diff --git a/yasm_compile.gypi b/yasm_compile.gypi
index 0d9263f..58a0dfb 100644
--- a/yasm_compile.gypi
+++ b/yasm_compile.gypi
@@ -6,9 +6,11 @@
 #
 # Files to be compiled with YASM should have an extension of .asm.
 #
-# There are two variables for this include:
+# There are three variables for this include:
 # yasm_flags : Pass additional flags into YASM.
 # yasm_output_path : Output directory for the compiled object files.
+# yasm_includes : Includes used by .asm code.  Changes to which should force
+#                 recompilation.
 #
 # Sample usage:
 # 'sources': [
@@ -19,6 +21,7 @@
 #     '-I', 'assembly_include',
 #   ],
 #   'yasm_output_path': '<(SHARED_INTERMEDIATE_DIR)/project',
+#   'yasm_includes': ['ultra_optimized_awesome.inc']
 # },
 # 'includes': [
 #   'third_party/yasm/yasm_compile.gypi'
@@ -27,6 +30,7 @@
 {
   'variables': {
     'yasm_flags': [],
+    'yasm_includes': [],
 
     'conditions': [
       [ 'use_system_yasm==0', {
@@ -98,7 +102,7 @@
     {
       'rule_name': 'assemble',
       'extension': 'asm',
-      'inputs': [ '<(yasm_path)', ],
+      'inputs': [ '<(yasm_path)', '<@(yasm_includes)'],
       'outputs': [
         '<(yasm_output_path)/<(RULE_INPUT_ROOT).<(asm_obj_extension)',
       ],