1There are two separate wrappers for V8 here.  One is called FXJS, and
2it is used by the non-XFA code.  The other is called FXJSE, and it is
3used only by the XFA code.  Additionally FXJSE may request services
4from FXJS to bridge the two.
5
6Both the FXJS and FXJSE binding code needs to be replaced by something
7saner, perhaps Gin or perhaps some IDL. See
8  https://bugs.chromium.org/p/pdfium/issues/detail?id=716
9for progress on the issue.
10
11FXJS binds objects by sticking a pointer to a CFXJS_PerObjectData in
12the V8 object's internal slot.  FXJSE binds objects by sticking a
13pointer to either an actual v8 function object or a CFXJSE_HostObject
14in the V8 object's internal slot, depending upon whether the object
15represents (in some notion) a "class" or an "instance". Also, V8 objects
16bound in one library may unexpectedly arrive at the other given a script
17that's trying to mess with us.
18
19To distinguish these cases, we use two internal slots for all bound
20objects, regardless of the FXJS/FXJSE distinction.  Slot 0 is the
21tag and contains either:
22  kPerObjectDataTag for FXJS objects, or
23  g_FXJSETagString for FXJSE Host objects, or
24  One of 4 specific FXJSE_CLASS_DESCRIPTOR globals for FXJSE classes:
25    GlobalClassDescriptor
26    NormalClassDescriptor
27    VariablesClassDescriptor
28    formcalc_fm2js_descriptor
29
30Slot 1's contents are determined by these tags:
31  kPerObjectDataTag means to expect a CFXJS_PerObjectData.
32  g_FXJSETagString means to expect a CFXJSE_HostObject.
33  A FXJSE_CLASS_DESCRIPTOR pointer means to expect a v8 function.
34