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This change adds a @IdentifierNameString annotation to use the
-identifiernamestring R8 rule. There are a few limitations to this:
  - -identifiernamestring only obfuscates the string, it does not
    actually check that the class exists.
  - If a field has the annotation, it must be non-final, otherwise javac
    will inline the constant and R8 won't obfuscate it.
  - Any field/method must be assigned/called with a String literal,
    otherwise R8 won't obfuscate the string and there will be issues at
    runtime.

Due to these limitations, I ended up adding a getIdentifierName() method
to SplitCompatUtils which returns the obfuscated name of implementation
classes for split compat. I also added a PRESUBMIT check to enforce that
only string literals are passed to that method. There can be cases where
R8 will not fail, but a string variable is silently ignored if it is not
a String literal.

Bug: 1126301
Change-Id: Ie5216e6a81cdbca71cc117460704ea4d9122fc78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2451388
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814212}