OTS: Add GSUB table support.

- Added src/gsub.{cc,h} for supporting GSUB table.
- Many fonts don't arrange script and feature tags in alphabetical order and
it might not be a security issue so no longer treat it as an error.

BUG=27131
TEST=http://code.google.com/p/ots/wiki/HowToTestOts (Verified with ~4000 TrueType/OpenType fonts) 
TEST=Confirmed that chrome can render the W3C font tests[1] for TTF font linking of Complex scripts(Khmer and Devanagari) as expected by hand.

[1] http://www.w3.org/International/tests/tests-html-css/list-fonts

ots-common.gypi

@@ -25,6 +25,8 @@
       'src/glyf.h',
       'src/gpos.cc',
       'src/gpos.h',
+      'src/gsub.cc',
+      'src/gsub.h',
       'src/hdmx.cc',
       'src/hdmx.h',
       'src/head.cc',

src/gdef.cc

@@ -7,6 +7,8 @@
 #include <limits>
 #include <vector>
 
+#include "gpos.h"
+#include "gsub.h"
 #include "layout.h"
 #include "maxp.h"
 
@@ -238,7 +240,7 @@ bool ParseMarkGlyphSetsDefTable(ots::OpenTypeFile *file, const uint8_t *data,
 }  // namespace
 
 #define DROP_THIS_TABLE \
-  do { delete file->gdef; file->gdef = 0; } while (0)
+  do { file->gdef->data = 0; file->gdef->length = 0; } while (0)
 
 namespace ots {
 
@@ -367,7 +369,11 @@ bool ots_gdef_parse(OpenTypeFile *file, const uint8_t *data, size_t length) {
 }
 
 bool ots_gdef_should_serialise(OpenTypeFile *file) {
-  return file->gdef != NULL;
+  const bool needed_tables_dropped =
+      (file->gsub && file->gsub->data == NULL) ||
+      (file->gpos && file->gpos->data == NULL);
+  return file->gdef != NULL && file->gdef->data != NULL &&
+      !needed_tables_dropped;
 }
 
 bool ots_gdef_serialise(OTSStream *out, OpenTypeFile *file) {

src/gpos.cc

@@ -7,6 +7,8 @@
 #include <limits>
 #include <vector>
 
+#include "gdef.h"
+#include "gsub.h"
 #include "layout.h"
 #include "maxp.h"
 
@@ -665,7 +667,7 @@ bool ParseExtensionPositioning(const ots::OpenTypeFile *file,
 }  // namespace
 
 #define DROP_THIS_TABLE \
-  do { delete file->gpos; file->gpos = 0; } while (0)
+  do { file->gpos->data = 0; file->gpos->length = 0; } while (0)
 
 namespace ots {
 
@@ -784,7 +786,11 @@ bool ots_gpos_parse(OpenTypeFile *file, const uint8_t *data, size_t length) {
 }
 
 bool ots_gpos_should_serialise(OpenTypeFile *file) {
-  return file->gpos != NULL;
+  const bool needed_tables_dropped =
+      (file->gdef && file->gdef->data == NULL) ||
+      (file->gsub && file->gsub->data == NULL);
+  return file->gpos != NULL && file->gpos->data != NULL &&
+      !needed_tables_dropped;
 }
 
 bool ots_gpos_serialise(OTSStream *out, OpenTypeFile *file) {

src/gpos.h

@@ -16,6 +16,7 @@ struct OpenTypeGPOS {
         length(0) {
   }
 
+  // Number of lookups in GPOS table
   uint16_t num_lookups;
 
   const uint8_t *data;