Disable --harmony-object-observe

This was previously reverted due to breakage in devtools, but that has
been worked around in https://codereview.chromium.org/1666573002.

The feature has been publicly-announced as deprecated for several months,
and Chrome 49 will emit deprecation warnings in the console for
uses of the API. This CL aims to remove it from M50 (which is what the
message warns of).

BUG=chromium:552100
LOG=y
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1711863003

Cr-Commit-Position: refs/heads/master@{#34171}

src/flag-definitions.h

@@ -205,6 +205,7 @@ DEFINE_IMPLICATION(es_staging, harmony_tailcalls)
 
 // Features that are still work in progress (behind individual flags).
 #define HARMONY_INPROGRESS(V)                                                \
+  V(harmony_object_observe, "harmony Object.observe")                        \
   V(harmony_modules, "harmony modules")                                      \
   V(harmony_function_sent, "harmony function.sent")                          \
   V(harmony_sharedarraybuffer, "harmony sharedarraybuffer")                  \
@@ -229,7 +230,6 @@ DEFINE_IMPLICATION(es_staging, harmony_tailcalls)
   V(harmony_default_parameters, "harmony default parameters")             \
   V(harmony_destructuring_assignment, "harmony destructuring assignment") \
   V(harmony_destructuring_bind, "harmony destructuring bind")             \
-  V(harmony_object_observe, "harmony Object.observe")                     \
   V(harmony_tostring, "harmony toString")                                 \
   V(harmony_regexps, "harmony regular expression extensions")             \
   V(harmony_unicode_regexps, "harmony unicode regexps")                   \