Avoid duplicated MockTSFBridge symbols

mock_tsf_bridge.cc was compiled as a part of //ui/base:test_support and
//ui/base/ime/win:win. This caused link errors in some configurations as
MockTSFBridge symbols were duplicated. //ui/base:test_support depends on
//ui/base/ime/win:win so mock_tsf_bridge.cc is redundant there.

Bug: 968222
Change-Id: Ibc159c336c0feef285e1066d3dc73c57c62b981c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2379764
Commit-Queue: Yohei Yukawa <yukawa@chromium.org>
Auto-Submit: Tomasz Moniuszko <tmoniuszko@opera.com>
Reviewed-by: Yohei Yukawa <yukawa@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Anupam Snigdha <snianu@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#803186}

ui/base/BUILD.gn

@@ -774,13 +774,6 @@ static_library("test_support") {
       "ime/dummy_text_input_client.h",
     ]
 
-    if (is_win) {
-      sources += [
-        "ime/win/mock_tsf_bridge.cc",
-        "ime/win/mock_tsf_bridge.h",
-      ]
-    }
-
     deps += [
       "//ui/base/ime",
       "//ui/events",

ui/base/ime/win/mock_tsf_bridge.h

@@ -16,7 +16,7 @@
 
 namespace ui {
 
-class MockTSFBridge : public TSFBridge {
+class COMPONENT_EXPORT(UI_BASE_IME_WIN) MockTSFBridge : public TSFBridge {
  public:
   MockTSFBridge();
   ~MockTSFBridge() override;