use longer-lived alloc in CacheKey()
use longer-lived alloc in CacheKey()

The gradient shader allocates data in the SkArenaAlloc
and points into it via its uniforms (as if it were a
bitmap or image), so we need to keep that alive while
it may be used.  The scope of CacheKey() is not long
enough.

When the cache is disabled or we get a cache miss,
everything was fine, since we were working out
of the long-lived fAlloc in Builder().

But when we hit something in the cache, we'd be left
trying to draw from data in this temporary scoped
SkArenaAlloc in CacheKey(), a use-after-free.

Change-Id: Ie873d57339180f4cd320c940a72aeb0563b93325
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/266356
Reviewed-by: Mike Reed <f5cabf8735907151a446812c9875d6c0c712d847@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia