Disable CSSAgentTest.GetSource

Disabling instead of reverting as the revert is not possible without
reverting a chain of changes.

https://chromium-review.googlesource.com/c/chromium/src/+/1750102

Disabling currently failing test.

Bug: 993097
TBR: weili@chromium.org
Change-Id: I88c176f1aa4fe64419e54b88ebb29d775a1e1574
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1748119Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Commit-Queue: Kristyn Hamasaki <khamasaki@google.com>
Cr-Commit-Position: refs/heads/master@{#686211}

components/ui_devtools/css_agent_unittest.cc

@@ -316,7 +316,7 @@ TEST_F(CSSAgentTest, UpdateOnBoundsChange) {
   EXPECT_EQ(2, GetStyleSheetChangedCount(another_element_stylesheet_id));
 }
 
-TEST_F(CSSAgentTest, GetSource) {
+TEST_F(CSSAgentTest, DISABLED_GetSource) {
   std::string file = "components/test/data/ui_devtools/test_file.cc";
   element()->AddSource(file, 0);
   auto result = GetSourceForElement();