[LiteRT-LM][ConversationAPI] Update indexmap to be usable from 1st party code.

Indexmap will be used by ANTLR parser to construct ordered json.

ATL discussion thread:
https://groups.google.com/a/google.com/g/chrome-atls-discuss/c/o6MBsEjjky8/m/1uX2rd-zDwAJ

Bug: 467777835
Change-Id: Id3ed5f7b96e7df76f39ec28768db8fd71a67b36e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7396687
Commit-Queue: Jingyun Liu <jingyun@google.com>
Reviewed-by: Robbie McElrath <rmcelrath@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1565091}
NOKEYCHECK=True
GitOrigin-RevId: 9459cbd7145124c63d3b67e727c2226382e8930e

Author: jingyun19

chromium_crates_io/Cargo.lock

@@ -26,6 +26,7 @@ fend-core = "1"
 font-types = "0.10"
 hex = "0.4"
 hmac-sha256 = "1.1.12"
+indexmap = "2.12.1"
 jxl = "0.2.2"
 lazy_static = "1"
 libc = "0.2"

chromium_crates_io/Cargo.toml

@@ -62,11 +62,6 @@ cargo_crate("lib") {
     "std",
   ]
 
-  # Only for usage from third-party crates. Add the crate to
-  # //third_party/rust/chromium_crates_io/Cargo.toml to use
-  # it from first-party code.
-  visibility = [ "//third_party/rust/*" ]
-
   #####################################################################
   # Tweaking which GN `config`s apply to this target.