Convert source sets to static libraries.

This improves build performance somewhat. This does not change the standalone
targets since these don't affect the build time of Chrome and are small enough
for it not to matter anyway.

BUG=http://crbug.com/627637

Review-Url: https://codereview.chromium.org/2164633005
diff --git a/BUILD.gn b/BUILD.gn
index f622f05..f824163 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -147,7 +147,7 @@
   }
 }
 
-source_set("test_support") {
+static_library("test_support") {
   testonly = true
   sources = [
     "testing/fx_string_testhelpers.cpp",
diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index 766b676..03f11c1 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -16,7 +16,7 @@
   configs = [ "..:pdfium_common_config" ]
 }
 
-source_set("bigint") {
+static_library("bigint") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
@@ -106,7 +106,7 @@
     }
   }
 
-  source_set("fx_agg") {
+  static_library("fx_agg") {
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [
       "//build/config/compiler:no_chromium_code",
@@ -149,7 +149,7 @@
   }
 }
 
-source_set("fx_lcms2") {
+static_library("fx_lcms2") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
@@ -197,7 +197,7 @@
   }
 }
 
-source_set("jpeg") {
+static_library("jpeg") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
@@ -275,7 +275,7 @@
   }
 }
 
-source_set("fx_libopenjpeg") {
+static_library("fx_libopenjpeg") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
@@ -307,7 +307,7 @@
   ]
 }
 
-source_set("fx_lpng") {
+static_library("fx_lpng") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
@@ -341,7 +341,7 @@
 }
 
 if (pdf_enable_xfa) {
-  source_set("fx_tiff") {
+  static_library("fx_tiff") {
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [
       "//build/config/compiler:no_chromium_code",
@@ -405,7 +405,7 @@
   }
 }
 
-source_set("fx_zlib") {
+static_library("fx_zlib") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
@@ -433,6 +433,7 @@
   ]
 }
 
+# Can not be a static library due to lack of .cc files.
 source_set("pdfium_base") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [