Summary:
While in theory we should never delete views before removing them from the hierarchy, there are some exceptions:

(1) Some mysterious cases that don't seem like bugs, but where the child still seems to keep a reference to the parent:
(2) When deleting views as part of stopSurface.

On #1: in the past we had issues when we assumed that ViewManager.getChildCount() would return an accurate count. Sometimes it's just... wrong. Here, I've found at least one case where a View still has a parent after it's removed from the View hierarchy. I assume this is undocumented Android behavior or an Android bug, but either way, there's nothing I can do about it.

On #2: there are valid cases where we want to delete a View without explicitly removing it from the View hierarchy (it will eventually be removed from the hierarchy when the Root view is unmounted, but it may still be "in" a View hierarchy when it's deleted).

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D22321374

fbshipit-source-id: 9667bbe778c418f0216550638dc26ca48a58e5fa

Summary:
changelog: [internal]

Prevents 2 type converions:
1. int <-> size_t
2. int <-> int32_t

# Why is using size_t better when working with indexes.

## 1. Type conversion isn't for free.

Take this example

```
size_t calculate(int number) {
  return number + 1;
}
```

It generates following assembly (generated with armv8-a clang 10.0.0):

```
calculate(int):                          // calculate(int)
sub     sp, sp, facebook#16                     // =16
str     w0, [sp, facebook#12]
ldr     w8, [sp, facebook#12]
add     w9, w8, #1                      // =1
mov     w8, w9
sxtw    x0, w8
add     sp, sp, facebook#16                     // =16
ret
```

That's 9 instructions.

If we get rid of type conversion:

```
size_t calculate(size_t number) {
  return number + 1;
}
```

Assembly (generated with armv8-a clang 10.0.0):

```
calculate(unsigned long):                          // calculate(unsigned long)
sub     sp, sp, facebook#16             // =16
str     x0, [sp, facebook#8]
ldr     x8, [sp, facebook#8]
add     x0, x8, #1              // =1
add     sp, sp, facebook#16             // =16
ret
```

Compiler now produces only 7 instructions.

## Semantics

When using int for indexing, the type doesn't say much. By using `size_t`, just by looking at the type, it gives the reader more information about where it is coming from.

Reviewed By: JoshuaGross

Differential Revision: D24332248

fbshipit-source-id: 87ef982829ec14906ed9e002ea2e875fda4a0cd8