Add FORM_OnLButtonDoubleClick().

The code for this already exists, but is not hooked up to a public API.
Hook it up and add a test case.

Change-Id: I2ebc8492d8b7347849ff06f664155c6d72ecf76f
Reviewed-on: https://pdfium-review.googlesource.com/c/44130
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/fpdfsdk/cpdfsdk_annothandlermgr.cpp b/fpdfsdk/cpdfsdk_annothandlermgr.cpp
index ac22fe2..7d253c8 100644
--- a/fpdfsdk/cpdfsdk_annothandlermgr.cpp
+++ b/fpdfsdk/cpdfsdk_annothandlermgr.cpp
@@ -145,6 +145,16 @@
       ->OnLButtonUp(pPageView, pAnnot, nFlags, point);
 }
 
+bool CPDFSDK_AnnotHandlerMgr::Annot_OnLButtonDblClk(
+    CPDFSDK_PageView* pPageView,
+    CPDFSDK_Annot::ObservedPtr* pAnnot,
+    uint32_t nFlags,
+    const CFX_PointF& point) {
+  ASSERT(pAnnot->HasObservable());
+  return GetAnnotHandler(pAnnot->Get())
+      ->OnLButtonDblClk(pPageView, pAnnot, nFlags, point);
+}
+
 bool CPDFSDK_AnnotHandlerMgr::Annot_OnMouseMove(
     CPDFSDK_PageView* pPageView,
     CPDFSDK_Annot::ObservedPtr* pAnnot,