[Impeller] Don't define CanvasRecorder if IMPELLER_TRACE_CANVAS is not set. (flutter/engine#46476)

Internal bug: b/303067268

https://github.com/flutter/engine/pull/46376 is causing a breakage to the internal engine build because of 543348a044/impeller/aiks/canvas_recorder.h (L58-L62). Internally, we do not set `IMPELLER_TRACE_CANVAS`.

It looks like the cause is that the internal toolchain causes the `static_assert` to be compiled even though the template is not instantiated.

@chingjun helped me to figure out the following:

https://stackoverflow.com/questions/5246049/c11-static-assert-and-template-instantiation points us to the spec. In the later version (ISO/IEC 14882:2017(E)):

> The program is ill-formed, no diagnostic required, if ... no valid specialization can be generated for a template or a substatement of a constexpr if statement (9.4.1) within a template and the template is not instantiated,

<details>

<summary>The relevant section</summary>

![The relevant section of the spec](https://github.com/flutter/engine/assets/7111741/4503efcd-9479-4c7a-b4a1-7302dea1653b)

</details>

Interpretation: the compiler can either choose to emit the error caused by the `static_assert` or not. Currently the compiler used by the build here on LUCI does not; internally it does.

For example, the following links shows that simply changing the Clang version affects whether the error appears or not for a minimal template.

- ok: https://godbolt.org/z/n9nYrcvcP
- not ok: https://godbolt.org/z/fWcvdcn35

Hence, `#ifdef` out the class instead of using a `static_assert` for more consistent behavior across these two toolchains.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/impeller/aiks/canvas_recorder.h

@@ -47,6 +47,9 @@ enum CanvasRecorderOp : uint16_t {
   DrawAtlas,
 };
 
+// Canvas recorder should only be used when IMPELLER_TRACE_CANVAS is defined
+// (never in production code).
+#ifdef IMPELLER_TRACE_CANVAS
 /// Static polymorphic replacement for impeller::Canvas that records methods
 /// called on an impeller::Canvas and forwards it to a real instance.
 /// TODO(https://github.com/flutter/flutter/issues/135718): Move this recorder
@@ -55,12 +58,6 @@ enum CanvasRecorderOp : uint16_t {
 template <typename Serializer>
 class CanvasRecorder {
  public:
-#ifndef IMPELLER_TRACE_CANVAS
-  // Canvas recorder should only be used when IMPELLER_TRACE_CANVAS is defined
-  // (never in production code).
-  static_assert(false);
-#endif
-
   CanvasRecorder() : canvas_() { serializer_.Write(CanvasRecorderOp::New); }
 
   explicit CanvasRecorder(Rect cull_rect) : canvas_(cull_rect) {
@@ -286,5 +283,6 @@ class CanvasRecorder {
   Canvas canvas_;
   Serializer serializer_;
 };
+#endif
 
 }  // namespace impeller