Use C99 for size_t loops.

This was done just by grepping for 'size_t i;' and 'size_t j;'. I left
everything in crypto/x509 and friends alone.

There's some instances in gcm.c that are non-trivial and pulled into a
separate CL for ease of review.

Change-Id: I6515804e3097f7e90855f1e7610868ee87117223
Reviewed-on: https://boringssl-review.googlesource.com/10801
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/ssl_cipher.c

diff --git a/ssl/ssl_cipher.c b/ssl/ssl_cipher.c
index e58d889..ea274ad 100644
--- a/ssl/ssl_cipher.c
+++ b/ssl/ssl_cipher.c
@@ -1047,8 +1047,7 @@
   /* The set of ciphers is static, but some subset may be unsupported by
    * |ssl_method|, so the list may be smaller. */
   size_t co_list_num = 0;
-  size_t i;
-  for (i = 0; i < kCiphersLen; i++) {
+  for (size_t i = 0; i < kCiphersLen; i++) {
     const SSL_CIPHER *cipher = &kCiphers[i];
     if (ssl_method->supports_cipher(cipher)) {
       co_list[co_list_num].cipher = cipher;
@@ -1067,7 +1066,7 @@
     if (co_list_num > 1) {
       co_list[0].next = &co_list[1];
 
-      for (i = 1; i < co_list_num - 1; i++) {
+      for (size_t i = 1; i < co_list_num - 1; i++) {
         co_list[i].prev = &co_list[i - 1];
         co_list[i].next = &co_list[i + 1];
       }