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/custom_extensions.c

diff --git a/ssl/custom_extensions.c b/ssl/custom_extensions.c
index 4a9baa8..ed802ee 100644
--- a/ssl/custom_extensions.c
+++ b/ssl/custom_extensions.c
@@ -32,8 +32,7 @@
 static const SSL_CUSTOM_EXTENSION *custom_ext_find(
     STACK_OF(SSL_CUSTOM_EXTENSION) *stack,
     unsigned *out_index, uint16_t value) {
-  size_t i;
-  for (i = 0; i < sk_SSL_CUSTOM_EXTENSION_num(stack); i++) {
+  for (size_t i = 0; i < sk_SSL_CUSTOM_EXTENSION_num(stack); i++) {
     const SSL_CUSTOM_EXTENSION *ext = sk_SSL_CUSTOM_EXTENSION_value(stack, i);
     if (ext->value == value) {
       if (out_index != NULL) {
@@ -69,8 +68,7 @@
     return 1;
   }
 
-  size_t i;
-  for (i = 0; i < sk_SSL_CUSTOM_EXTENSION_num(stack); i++) {
+  for (size_t i = 0; i < sk_SSL_CUSTOM_EXTENSION_num(stack); i++) {
     const SSL_CUSTOM_EXTENSION *ext = sk_SSL_CUSTOM_EXTENSION_value(stack, i);
 
     if (ssl->server &&