Bug 1444481 part 1: Add some needed #includes to ComputedTimingFunction.h and FrameMetrics.h. r=xidorn

(This is a helper patch -- I'm splitting this into its own patch since it's
changing files in other directories, and also so that the main patches here
can be a bit more direct.)

Without this change, the other patches in this series would cause compile
failures in the headers that I'm fixing up here (because the other patches will
be removing #includes from some headers that these files were inadvertently
depending on).

As of this patch, ComputedTimingFunction.h will now be including:
 - nsDebug.h to provide NS_ASSERTION
 - nsStringFwd.h to provide a forward-declaration for "nsAString&"
 - Assertions.h to provide MOZ_ASSERT
 - Maybe.h to provide Maybe<ComputedTimingFunction>
(I think it's been leaning on nsTimingFunction.h's include of nsString.h to
indirectly provide these.)

FrameMetrics.h will now be including:
 - PLDHashTable.h to provide PLDHashNumber
(I think it's been leaning on nsStyleCoord.h/nsStyleConsts.h to indirectly
provide this.)

MozReview-Commit-ID: AoFoEe9GisK

--HG--
extra : rebase_source : 63c69343acaf42511ebdeb0238f4385a0c6345a5

dom/animation/ComputedTimingFunction.h

@@ -7,9 +7,14 @@
 #ifndef mozilla_ComputedTimingFunction_h
 #define mozilla_ComputedTimingFunction_h
 
+#include "nsDebug.h"
 #include "nsSMILKeySpline.h"  // nsSMILKeySpline
+#include "nsStringFwd.h"
 #include "nsTimingFunction.h"
 
+#include "mozilla/Assertions.h"
+#include "mozilla/Maybe.h"
+
 namespace mozilla {
 
 class ComputedTimingFunction

gfx/layers/FrameMetrics.h

@@ -21,6 +21,7 @@
 #include "mozilla/TimeStamp.h"          // for TimeStamp
 #include "nsString.h"
 #include "nsStyleCoord.h"               // for nsStyleCoord
+#include "PLDHashTable.h"               // for PLDHashNumber
 
 namespace IPC {
 template <typename T> struct ParamTraits;