Code change to avoid signed/unsigned mismatch warnings.

This makes pdfium code on Linux and Mac sign-compare warning free.
The warning flag will be re-enabled after checking on windows clang build.

BUG=pdfium:29
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1841643002 .
diff --git a/fpdfsdk/fpdfsave_embeddertest.cpp b/fpdfsdk/fpdfsave_embeddertest.cpp
index 2b138a6..5dbff36 100644
--- a/fpdfsdk/fpdfsave_embeddertest.cpp
+++ b/fpdfsdk/fpdfsave_embeddertest.cpp
@@ -19,14 +19,14 @@
   EXPECT_TRUE(OpenDocument("hello_world.pdf"));
   EXPECT_TRUE(FPDF_SaveAsCopy(document(), this, 0));
   EXPECT_THAT(GetString(), testing::StartsWith("%PDF-1.7\r\n"));
-  EXPECT_EQ(843, GetString().length());
+  EXPECT_EQ(843u, GetString().length());
 }
 
 TEST_F(FPDFSaveEmbedderTest, SaveSimpleDocWithVersion) {
   EXPECT_TRUE(OpenDocument("hello_world.pdf"));
   EXPECT_TRUE(FPDF_SaveWithVersion(document(), this, 0, 14));
   EXPECT_THAT(GetString(), testing::StartsWith("%PDF-1.4\r\n"));
-  EXPECT_EQ(843, GetString().length());
+  EXPECT_EQ(843u, GetString().length());
 }
 
 TEST_F(FPDFSaveEmbedderTest, SaveSimpleDocWithBadVersion) {