Remove some unneeded includes in pdf/ unit tests.

Several files includes gmock.h but does not use GMock. Remove the
gmock.h include, or replace it with gtest.h.

Change-Id: Id1c1928365ff0124c80c0f7baaaa2388c530472a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1727471
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682771}

pdf/pdfium/accessibility_unittest.cc

@@ -9,7 +9,7 @@
 #include "pdf/pdfium/pdfium_test_base.h"
 #include "pdf/test/test_client.h"
 #include "ppapi/c/private/ppb_pdf.h"
-#include "testing/gmock/include/gmock/gmock.h"
+#include "testing/gtest/include/gtest/gtest.h"
 
 #if defined(OS_CHROMEOS)
 #include "base/system/sys_info.h"

pdf/pdfium/pdfium_engine_exports_unittest.cc

@@ -8,7 +8,6 @@
 #include "base/threading/thread_task_runner_handle.h"
 #include "gin/v8_initializer.h"
 #include "pdf/pdf.h"
-#include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/geometry/size.h"