Add comments for two test expectations and move them to the right place.

webkit/tools/layout_tests/test_expectations.txt

Index: webkit/tools/layout_tests/test_expectations.txt
===================================================================
--- webkit/tools/layout_tests/test_expectations.txt	(revision 29489)
+++ webkit/tools/layout_tests/test_expectations.txt	(working copy)
@@ -1126,6 +1126,16 @@
 // alpha in bottom group.
 BUG10310 LINUX : LayoutTests/svg/W3C-SVG-1.1/masking-opacity-01-b.svg = FAIL
 
+// Image mismatch: some green blocks are not displayed.
+// Started to fail WebKit upstream change r49757, and probably related to it.
+BUG25175 : LayoutTests/svg/W3C-SVG-1.1/masking-mask-01-b.svg = IMAGE
+
+// Image mismatch since WebKit r49757.
+// The difference is hard to tell, but our gradation is slightly different
+// (blue is too strong on the gradation).
+// Pixel dump was updated on upstream but our rendering did not change.
+BUG25175 MAC : LayoutTests/svg/custom/grayscale-gradient-mask.svg = IMAGE
+
 //
 // -----------------------------------------------------------------
 // End of SVG tests
@@ -3162,8 +3172,6 @@
 
 // WebKit Roll 49684:49761
 BUG_YUTAK : LayoutTests/fast/events/drag-and-drop-dataTransfer-types-nocrash.html = FAIL
-BUG_YUTAK : LayoutTests/svg/W3C-SVG-1.1/masking-mask-01-b.svg = FAIL
-BUG_YUTAK MAC : LayoutTests/svg/custom/grayscale-gradient-mask.svg = FAIL
 BUG_YUTAK LINUX : LayoutTests/http/tests/security/mixedContent/insecure-plugin-in-iframe.html = FAIL
 
 // WebKit Roll 49761:49809