Add support for the Metal backend on all iOS builds.

[chinmaygarde]

Metal is not enabled by default yet. This commit will increase the size of the
iOS engine binary. A breakdown of the increase is as follows:

Both binaries are Mach-O 64-bit dynamically linked shared library of type arm64.
For reproduction, these numbers were gathered at Engine hash
01a52b9.

• Uncompressed Metal iOS Engine Version: 11380 KB
• Uncompressed Non-Metal iOS: 11128 KB

Detailed breakdown of size differences:

     VM SIZE                                FILE SIZE
 --------------                          --------------
  +2.0%  +106Ki __TEXT,__text             +106Ki  +2.0%
  +2.6% +70.9Ki String Table             +70.9Ki  +2.6%
  +2.3% +35.5Ki Symbol Table             +35.5Ki  +2.3%
  +171% +8.32Ki [__DATA]                 +8.32Ki +13e3%
  +2.3% +6.48Ki __TEXT,__cstring         +6.48Ki  +2.3%
  +2.3% +6.12Ki __DATA,__const           +6.12Ki  +2.3%
   +23% +4.67Ki [__TEXT]                 +4.67Ki   +23%
  +109% +4.06Ki __TEXT,__gcc_except_tab  +4.06Ki  +109%
   +38% +3.98Ki [__LINKEDIT]                   0  [ = ]
   +18% +3.40Ki __TEXT,__objc_methname   +3.40Ki   +18%
  +3.5% +1.64Ki __TEXT,__unwind_info     +1.64Ki  +3.5%
   +23% +1.12Ki __DATA,__objc_selrefs    +1.12Ki   +23%
  +0.1%    +656 __TEXT,__const              +656  +0.1%
  +2.1%    +568 Function Start Addresses    +568  +2.1%
  +6.0%    +528 Binding Info                +528  +6.0%
  +2.1%    +320 Lazy Binding Info           +320  +2.1%
  +1.3%    +292 [5 Others]                  +292  +1.3%
  +4.5%    +288 __DATA,__cfstring           +288  +4.5%
  +3.8%    +160 [Mach-O Headers]            +160  +3.8%
  +1.2%    +136 Rebase Info                 +136  +1.2%
   +12%     +88 __DATA,__objc_classrefs      +88   +12%
  +2.3%  +256Ki TOTAL                     +252Ki  +2.3%

Fixes flutter/flutter#52369

[chinmaygarde]

https://chromium-review.googlesource.com/c/chromium/tools/build/+/2097397 needs to land before this so that LUCI doesn't end up trying to build a variant that does not exist.

[chinmaygarde]

The recipe update has landed.

[hy9be]

This may not be the right thread but I wonder if anyone plans to update the statement Incremental reductions to this size will continue to be made but a significant reduction requires the removal of the OpenGL backend. This is not something we have considered yet. in the wiki page https://github.com/flutter/flutter/wiki/Metal-on-iOS-FAQ, after #37448 is merged. Thanks.

[chinmaygarde]

You're right, I filed flutter/flutter#115243 to address this. IMO, we should just remove the entry.