Set |pcRunInfoRet| to 0 if |pcchPlainRet| is 0 in |ITextStoreACP::GetText|

TSF on Windows has an assumption that |pcchPlainRet| to be 0 if focused
edit control has no text(empty). Failure to satisfy the assumption may
cause hung (dead looped) in the application when using some legacy IMEs.
Therefore, we should set |pcchPlainRet| to 0 if there is no text in
focused edit control (|pcchPlainRet| is 0).

Bug: 981667
Change-Id: I0e851e9c82e7716c5434cfd8ca93eb1a091d0b79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1694408
Commit-Queue: Siye Liu <siliu@microsoft.com>
Reviewed-by: Yohei Yukawa <yukawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676183}

ui/base/ime/win/tsf_text_store.cc

@@ -263,10 +263,12 @@ STDMETHODIMP TSFTextStore::GetText(LONG acp_start,
     text_buffer[i] = result[i];
   }
 
-  if (run_info_buffer_size) {
+  if (*text_buffer_copied > 0 && run_info_buffer_size) {
     run_info_buffer[0].uCount = *text_buffer_copied;
     run_info_buffer[0].type = TS_RT_PLAIN;
     *run_info_buffer_copied = 1;
+  } else {
+    *run_info_buffer_copied = 0;
   }
 
   *next_acp = acp_end;

ui/base/ime/win/tsf_text_store_unittest.cc

@@ -273,10 +273,14 @@ class TSFTextStoreTestCallback {
     ASSERT_EQ(expected_string.size(), text_buffer_copied);
     EXPECT_EQ(expected_string,
               base::string16(buffer, buffer + text_buffer_copied));
-    EXPECT_EQ(1u, run_info_buffer_copied);
-    EXPECT_EQ(expected_string.size(), run_info.uCount);
-    EXPECT_EQ(TS_RT_PLAIN, run_info.type);
-    EXPECT_EQ(expected_next_acp, next_acp);
+    if (text_buffer_copied > 0) {
+      EXPECT_EQ(1u, run_info_buffer_copied);
+      EXPECT_EQ(expected_string.size(), run_info.uCount);
+      EXPECT_EQ(TS_RT_PLAIN, run_info.type);
+      EXPECT_EQ(expected_next_acp, next_acp);
+    } else {
+      EXPECT_EQ(0u, run_info_buffer_copied);
+    }
   }
 
   void GetTextErrorTest(LONG acp_start, LONG acp_end, HRESULT error_code) {